Apparatus and method for transmitting or receiving an uncompressed packet followed by compressed packets

ABSTRACT

A data transmission apparatus for sequentially transmitting data in units of packets each containing transmission data to the receiving end, comprises: a reception unit for receiving the transmission data as an input signal; a packet formation unit for receiving the transmission data received, and forming an uncompressed packet in which predetermined transmission data is stored as uncompressed data, and a compressed packet in which at least a portion of transmission data that follows the predetermined transmission data is compressed and stored as compressed data.

FIELD OF THE INVENTION

The present invention relates to a data transmission method, a datatransmission apparatus, a data reception apparatus, and a packet datastructure. More particularly, the invention relates to packet-by-packetdata transmission among plural data processing apparatuses, in whichdata corresponding to predetermined packets are compressed at thetransmitting end, and the compressed data corresponding to these packetsare restored at the receiving end.

BACKGROUND OF THE INVENTION

As representative transmission protocols for transmitting data on theInternet, TCP/IP (Transmission Control Protocol/Internet Protocol) andUDP/IP (User Datagram Protocol/Internet Protocol) are currently used.

However, when transmitting data in packet units by utilizing thesetransmission protocols through transmission paths having low˜medium bitrates (9600 bps˜64 Kbps), each packet includes headers corresponding tothe respective transmission protocols (i.e., TCP, UDP, and IP), andthese headers result in communication overhead, that is, the dataquantity in the header section of the packet becomes significantlylarger than the data quantity in the data section.

For example, when transmitting 10-byte data by UDP/IP, although the sizeof the data section of an UDP/IP packet used for this transmission isonly 10 bytes, the total size of the UDP/IP packet becomes 38 bytes. Inother words, the total size of the UDP/IP packet is about four times aslarge as the quantity of data which is actually transmitted. When suchcommunication overhead occurs frequently, the effective transmissionrate of data in the transmission path is significantly reduced.

As a method for reducing communication overhead due to use of pluraltransmission protocols, a header compression method proposed by V.Jacobson, which is defined in RFC (Request For Comments) 1144 and RFC2508, is currently used.

Hereinafter, this header compression method will be described.

FIG. 28(a) illustrates a data transmission system Cs1 to which the V.Jacobson's header compression method is applied.

In the data transmission system Cs1, a gateway server Sga is connectedto the Internet In, and a terminal equipment Teq such as a personalcomputer is connected to the gateway server Sga through a cable linesuch as a modem, ISDN (Integrated Service Digital Network), or LAN(Local Area Network). The terminal equipment Teq and the gateway serverSga are directly connected through the cable line by point-to-pointconnection. On the transmission path between the terminal equipment Tegand the gateway server Sga, packets which have been subjected to datacompression by the V. Jacobson's header compression method aretransmitted by a transmission protocol such as PPP (Point to PointProtocol).

In the data transmission system Cs1, when data is transmitted from thegateway server Sga to the terminal equipment Teq, the gateway server Sgaserves as a transmitter while the terminal equipment Teq serves as areceiver. On the contrary, when data is transmitted from the terminalequipment Teq to the gateway server Sg, the gateway server serves as areceiver while the terminal equipment Teq serves as a transmitter.

Hereinafter, a brief description will be given of data transmissionusing packets in the above-described data transmission system Cs1.

FIG. 28(b) illustrates processes requited for data transmission in thedata transmission system Cs1, corresponding to a plurality of layers inhierarchy. FIGS. 29(a)˜29(e) illustrate data structures of packets to begenerated by the processes corresponding to the respective layers.

FIG. 28(b) shows the case where data is transmitted from a server(transmitter) Sin on the Internet In through a gateway server (relayunit) Sga to a terminal equipment (receiver) Teq (refer to data flowDf).

Data stored in the transmitter Sin is subjected to the process of thefirst layer (application layer) Ls1, followed by the process of thesecond layer Ls2. Thereby, an RTP packet Prtp corresponding to RTP(Realtime Transport Protocol) is generated. This RTP packet is composedof an RTP header Hrtp containing header information and an RTP payload(data section) Drtp containing the above-described data. The informationin the RTP header Hrtp is composed of a sequence number Isn whichincrements by 1 every time one packet is transmitted, a time stamp Itswhich is used for processing at the data receiving end, and other headerinformation Ith (refer to FIG. 29(a)). The size of the RTP header Hrtpis generally 12 bytes, and 2 bytes are assigned to the sequence numberIsn and 4 bytes are assigned to the time stamp Its.

Next, the RTP packet Prtp is subjected to the process of the third layerLs3, whereby an UDP packet corresponding to UDP (user Datagram Protocol)is generated. This UDP packet Pudp is composed of an UDP header Hudpcontaining header information, and a data section Dudp containing theRTP packet Prtp (refer to FIG. 29(b)).

Subsequently, the UDP packet Pudp is subjected to the process of thefourth layer Ls4, whereby an IP packet Pipa corresponding to IP(Internet Protocol) is generated. This IP packet Pipa is composed of anIP header Hipa containing header information, and a data section Dipcontaining the UDP packet Pudp (refer to FIG. 29(c)).

Then, the IP packet Pipa is sent to the gateway server Sga through thetransmission path Cin, according to a predetermined transmissionstandard (e.g., Ethernet), by the process of the fifth layer Ls5.

In the gateway server Sga, the IP packet Pipa transmitted from theserver Sin on the Internet In is received according to the predeterminedtransmission standard (e.g., Ethernet) by the process of the lower layerLin2 which corresponds to the process of the fifth layer Ls5 in thetransmitter. The received IP packet Pipa is separated into the headerHipa and the data section Dip by the process of the upper layer Lin1which corresponds to the process of the fourth layer Ls4 in thetransmitter. Then, by the process of the upper layer Leq1 whichcorresponds to a predetermined layer in the receiver (in this case, thefourth layer Lr4), the separated data section Dip is given a headersection Hipb which includes information different from the informationstored in the header section Hipa, thereby generating an IP packet Pipb(refer to FIG. 29(d)).

Thereafter, the IP packet Pipb is subjected to the process of the lowerlayer Leq2 which corresponds to a predetermined layer in the receiver(in this case, the fifth layer Lr5), whereby a PPP packet Ppppcorresponding to PPP (Point to Point Protocol) is transmitted to theterminal equipment Teq through the cable line Ceq. This PPP packet Ppppis composed of a header section Hppp containing PPP header informationIppp and a CRC code Icrc for checking the received data, and a datasection Dppp containing the IP packet Pipb (refer to FIG. 29(e)).

When the PPP packet Pppp is received by the terminal equipment Teqserving as a receiver, the PPP packet Pppp is separated into the headersection Hppp and the data section Dppp, by the process of the fifthlayer Lr5 which corresponds to the lower layer Leq2 in the gatewayserver Sga.

Next, the Ip packet Pipb stored in the PPP data section Dppp isseparated into the IP header section Hipb and the IP data section Dip,by the process of the fourth layer Lr4 which is upper than the fifthlayer Lr5. Subsequently, the UDP packet Pudp stored in the IP datasection Dip is separated into the UDP header section Hudp and the UDPdata section Dudp by the process of the third layer Lr3 which is upperthan the fourth layer Lr4. Further, the RTP packet Prtp stored in theUDP data section Dudp is separated into the RTP header section Hrtp andthe RTP data section Drtp, by the process of the second layer Lr2 whichis upper than the third layer Lr3.

Then, the data stored in the RTP payload (RTP data section) Drtp issubjected to the process of the first layer (application layer) Lr1.

When data is transmitted from the terminal equipment Teq to the serverSin on the Internet, the terminal equipment Teq servers as a transmitterand the server Sin serves as a receiver. This case is reverse of thecase where data is transmitted from the server Sin on the Internetthrough the gateway server Sga to the terminal equipment Teq. That is,in the respective layers Lr2˜Lr5 of the terminal equipment Teq, packetscorresponding to these layers are generated. In the gateway server Sga,the IP packet Pipb is extracted from the PPP packet Pppp supplied fromthe terminal equipment Teg, and this packet is converted to the IPpacket Pipa to be sent to the server Sin on the Internet In on the basisof the Ethernet. In the server Sin, the IP packet Pipa is received bythe process of the fifth layer Ls5, and the data stored in the RTPpayload Drtp is taken out by the processes of the layers Ls4˜Ls2 in theserver Sin which correspond to the layers Lr2˜Lr4 in the terminalequipment Teq, and this data is subjected to the process of theapplication layer Ls1.

In the above description, an UDP packet corresponding to UDP isgenerated in the processes of the third layers Ls3 and Lr3, but a TCPpacket corresponding to TCP (Transmission Control Protocol) may begenerated in the processes of the third layers.

In the above-described data transmission system Cs1, when transmittingdata by PPP using the V. Jacobson's header compression method, two typesof packets are used as the PPP packets Pppp to be transmitted by thisprotocol, as shown in FIGS. 30(a) and 30(b). That is, one is acompressed packet Py in which data to be transmitted (hereinafterreferred to as transmission data) stored in the data section iscompressed (refer to FIG. 30(b)), and the other is an uncompressedpacket Px in which transmission data stored in the data section is notcompressed (refer to FIG. 30(a)). FIGS. 30(a) and 30(b) show only partsof these PPP packets, which are required for describing the V.Jacobson's header compression method.

That is, the uncompressed packet Px is composed of a header section Hpxcontaining header information, and a data section Dpx containingtransmission data (D) as uncompressed data Ir to be transmitted by PPP.The information in the header section Hpx is composed of acompression/uncompression identifier Ih1 which indicates whether thedata in the data section Dpx is compressed, and other header informationIh3. In the uncompressed packet Px, the identifier Ih1 indicates“uncompressed”.

Further, the compressed packet Py is composed of a header section Hpycontaining header information, and a data section Dpy containingdifference data (ΔD) as compressed data Id to be transmitted by PPP. Theinformation in the header section Hpy is composed of acompression/uncompression identifier Ih1 which indicates whether thedata stored in the data section Dpy is compressed, and other headerinformation Ih3. In the compressed packet Py, the identifier Ih1indicates “compressed”.

The header information Ih3 includes the CRC code Icrc shown in FIG.29(e).

In the above-described process of transmitting the PPP packets using theV. Jacobson's header compression method, the uncompressed packet Px istransmitted as the first PPP packet from the transmitting end to thereceiving end and, thereafter, the compressed packet Py is transmittedas the subsequent PPP packet.

In the data section Dpy of the compressed packet Py to be transmitted,difference data (ΔD) which is based on the transmission data (referencedata) of a PPP packet which has been transmitted just before thecompressed packet Py, is stored. To be specific, the difference data(ΔD) is a difference between the transmission data to be transmitted bythe compressed packet Py and the transmission data as the referencedata.

FIG. 31 is a diagram for conceptually explaining the PPP packettransmission process using the V. Jacobson's header compression method.

In FIG. 31, transmission data (D1)˜(D4) corresponding to the respectivePPP packets are sequentially transmitted.

Initially, an uncompressed packet Px(1) is transmitted as the first PPPpacket from the transmitting end to the receiving end. In the datasection Dpx of this uncompressed packet Px(1), transmission data (D1) isstored as uncompressed data Ir.

Thereafter, compressed packets Py(2)˜Py(4) are sequentially transmittedas PPP packets-from the transmitting end to the receiving end. In thedata sections Dpy of these compressed packets Py(2), Py(3), and Py(4),difference data (D1−D2), (D2−D3), and (D3−D4) are stored as compresseddata Id, respectively.

The difference data (D1−D2) is a difference between the transmissiondata (D1) and (D2), the difference data (D2−D3) is a difference betweenthe transmission data (D2) and (D3), and the difference data (D3−D4) isa difference between the transmission data (D3) and (D4). In this way,each compressed packet contains, as compressed data Id, difference databetween the transmission data of this compressed packet and thetransmission data of the packet which has been transmitted just beforethe compressed packet.

This transmission of compressed packets is continued until atransmission error occurs.

At the receiving end, the uncompressed packet Px(1) is ˜received as thefirst PPP packet and, thereafter, the compressed packets Py(2)˜Py(4) aresequentially received as PPP packets.

Then, the uncompressed packet Px(1) is processed according to PPP,whereby the transmission data (D1) stored in the data section Dpx istaken out. Further, the compressed packets Py(2), Py(3), and Py(4) areprocessed according to PPP, whereby the difference data (D1−D2),(D2−D3), and (D3−D4) stored in the data sections Dpy are taken out, andthe transmission data (D2), (D3), and (D4) corresponding to therespective compressed packets are restored by the V. Jacobson's headercompression method. For example, the transmission data (D2) is restoredby adding the difference data (D1−D2) and the transmission data (D1),and the transmission data (D3) and (D4) are restored in like manner.

Next, a description will be given of the case where a transmission erroroccurs during the PPP transmission process using the V. Jacobson'sheader compression method.

FIG. 32 illustrates exchange of data between the transmitting end andthe receiving end when a transmission error occurs.

When the receiving end detects that a transmission error has occurred ina predetermined PPP packet, since the receiving end cannot restore PPPpackets received after the occurrence of the transmission error, itnotifies the transmitting end that a restoration error has occurred.

For example, as shown in FIG. 32, in the case where the compressedpacket Py(2) is not normally transmitted to the receiving end due to atransmission error, even when the receiving end receives the compressedpacket Py(3) which follows the compressed packet Py(2), the transmittingend cannot restore the transmission data (D3) which is the original dataof the difference data stored in the compressed packet Py(3), becausethe transmission data (D2) which is the original data of the differencedata stored in the compressed packet Py(2) is not restored. Therefore,the receiving end notifies the transmitting end that a restoration errorhas occurred.

On receipt of the notification about the restoration error, thetransmitting end transmits an uncompressed packet Px(5) as a PPP packetto the receiving end. At the receiving end, all of the compressedpackets Py(2)˜Py(4) which have been received from when the transmissionerror occurred to when the uncompressed packet Px(5) is received, arediscarded.

By the way, in recent years, applications of the Internet using handyphones, such as mail access to handy phones and services of texts, haveproceeded. Further, infrastructure for next generation radiocommunication (˜384 Kbps) has been developed for practical use of thirdgeneration mobile communication (W-CDMA: Wideband-Code Division MultipleAccess).

FIG. 33(a) is a diagram illustrating a data transmission system Cs2employing a radio terminal adapted to W-CDMA.

In the data transmission system Cs2, a gateway server Sga is connectedto the Internet In, and a mobile radio terminal Tmo (e.g., visualterminal) is connected to the gateway server Sga through a wirelesstelephone network Cwr such as W-CDMA. Also in this data transmissionsystem Cs2, on the transmission path between the mobile radio terminalTmo and the gateway server Sga, packets which have been subjected todata compression based on the V. Jacobson's header compression methodare transmitted according to a protocol such as PPP (Point to PointProtocol).

FIG. 33(b) shows processes required for data transmission in the datatransmission system Cs2, corresponding to a plurality of layers inhierarchy.

In the data transmission system Cs2, since the transmission path betweenthe mobile radio terminal Tmo and the gateway server Sga includes thewireless telephone network Cwr, data transmission between them isdifferent in the following point from data transmission in the datatransmission system Cs1 in which the terminal Tmo and the server Sga areconnected by the cable line.

That is, when transmitting a PPP packet from the gateway server Sga tothe mobile radio terminal Tmo, the gateway server Sga transmits a PPPpacket obtained by the process of the second layer Lmo2, through theradio network Cmo, to the mobile radio terminal Tmo, by the process ofthe third layer Lmo3 which corresponds to the W-CDMA method.

The mobile radio terminal Tmo receives the PPP packet by the process ofthe sixth layer Lr6 which corresponds to the W-CDMA method. Thereafter,like the data transmission system Cs1, the data stored in the RTPpayload Drtp is taken out by the processes of the fifth to second layersLr5˜Lr2, and the data is subjected to the process of the applicationlayer Lr1.

Other processes for communication in the data transmission system Cs2shown in FIG. 33(b) are identical to those already described for thedata transmission system Cs1 shown in FIG. 28(b). For example, theprocesses of the first layer Lmo1 and the second layer Lmo2 at theterminal end in the gateway server Sga in the data transmission systemCs2 are identical to the processes of the first layer Leq1 and thesecond layer Leq2 at the terminal end in the gateway server Sga in thedata transmission system Cs1.

However, the bit error rate in data transmission in the radio section isabout 10⁻³ while the bit error rate in data transmission in the cablesection is 10⁻⁵˜10⁻⁷. Therefore, in the PPP data transmission using theV. Jacobson's header compression method (RFC1144, RFC2508), degradationof data quality due to transmission errors in the radio section becomesa problem.

In other words, in the data transmission system Cs2 including the radiosection, when data is transmitted by PPP using the V. Jacobson's headercompression method, the case shown in FIG. 32, i.e., discarding thereceived packets at the receiving end due to transmission errors, occursfrequently and, consequently, the number of transmitted packets to bediscarded increases considerably.

SUMMARY OF THE INVENTION

The present invention is made to solve the above-described problems andhas for its object to provide a data transmission method, a datatransmission apparatus, a data reception apparatus, and a packet datastructure, which can reduce the number of packets to be discarded at thereceiving end due to errors in the radio section, and thereby improvethe quality of data transmitted through the data transmission pathincluding the radio section, when performing packet-by-packet datatransmission with data (transmission data) stored in the packets beingcompressed.

Other objects and advantages of the invention will become apparent fromthe detailed description that follows. The detailed description andspecific embodiments described are provided only for illustration sincevarious additions and modifications within the scope of the inventionwill be apparent to those of skill in the art from the detaileddescription.

According to a first aspect of the present invention, there is provideda data transmission method for sequentially transmitting data in unitsof packets each containing transmission data, from the transmitting endto the receiving end. This method comprises a transmission-side processof transmitting an uncompressed packet in which predeterminedtransmission data is stored as uncompressed data, and then continuouslytransmitting a compressed packet in which at least a portion oftransmission data following the predetermined transmission data iscompressed and stored as compressed data; and a reception-side processof receiving the packets transmitted from the transmitting end, andrestoring the transmission data of the respective packets on the basisof the uncompressed data and the compressed data stored in therespective packets. The transmission-side process includes a compressionprocess of forming compressed data to be stored in a compressed packetto be transmitted, on the basis of the transmission data of a referencepacket that is the uncompressed packet and the transmission data of thecompressed packet to be transmitted. The reception-side process includesa restoration process of restoring the transmission data of a compressedpacket to be restored, on the basis of the transmission data of thereference packet and the compressed data included in the compressedpacket to be restored. Therefore, even when a transmission error occursin the compressed packet transmitted in the radio section, the receivingend can restore the subsequent compressed packet with reference to thetransmission data of the uncompressed packet as the reference packet.Thereby, the number of packets to be discarded at the receiving end dueto the transmission error in the radio section is reduced, with theresult that the quality of data transmitted in the transmission pathincluding the radio section is improved.

According to a second aspect of the present invention, in the datatransmission method of the first aspect, in the transmission-sideprocess, as the uncompressed packet, a packet including the uncompresseddata and a packet identifier indicating this packet is transmitted, andas the compressed packet that follows the uncompressed packet, a packetincluding the compressed data and a reference packet identifierindicating the uncompressed packet as a reference packet is transmitted.In the compression process, as the compressed data, difference databetween the transmission data of the reference packet and thetransmission data of the compressed packet is formed. Therefore, at thereceiving end, the reference packet which is required for restoration ofthe compressed packet can be specified according to the reference packetidentifier. Further, since the difference data between the transmissiondata of the reference packet and the transmission data of the compressedpacket is formed as the compressed data, the compressed packet to beincluded in the compressed packet can be formed by simple arithmeticprocessing.

According to a third aspect of the present invention, in the datatransmission method of the second aspect, in the transmission-sideprocess, additional information for calculating the difference data onthe basis of the transmission data of the reference packet is stored,and in the reception-side process, the difference data in the compressedpacket is calculated from the transmission data of the reference packet,on the basis of the additional information stored in the compressedpacket. Therefore, the data quantity of the difference data is reduced,and the data compression efficiency is improved, resulting in improveddata transmission efficiency.

According to a fourth aspect of the present invention, in the datatransmission method of the third aspect, the additional information is asequence number which indicates how many packets have been transmittedbefore the compressed packet, after transmission of the uncompressedpacket. Therefore, with respect to the transmission data which increasesby a predetermined quantity every time one packet is transmitted, thedifference data can be set to 0 bit.

According to a fifth aspect of the present invention, in the datatransmission method of the third aspect, the additional information is avariable of a calculation formula for calculating the difference data ofthe compressed packet from the transmission data of the referencepacket. Therefore, with respect to the transmission data which variesaccording to a predetermined function every time one packet istransmitted, the difference data can be reduced significantly.

According to a sixth aspect of the present invention, in the datatransmission method of the second aspect, in the transmission-sideprocess, a plurality of uncompressed packets which have been formed soas to be transmitted prior to the compressed packet are used asreference packets, and difference data between the transmission data ofeach reference packet and the transmission data of the compressed packetis associated with the reference packet identifier corresponding to eachreference packet, and plural sets of associated difference data andreference packet identifiers are stored in the compressed packet as thecompressed data. In the reception-side process, the transmission data ofthe compressed packet is restored using any set of different data andpacket identifier stored in the compressed packet. Therefore, thereliability of the transmission process for the uncompressed packetwhich includes information required for restoration of the compressedpacket, is improved, whereby the quality of data transmitted by radio issignificantly improved.

According to a seventh aspect of the present invention, in the datatransmission method of the first aspect, in the transmission-sideprocess, the uncompressed packet is transmitted at regular intervals.Therefore, the data size of the compressed packet is prevented fromsignificantly increasing, whereby the compression efficiency of thetransmission data is limited within an approximately constant variationrange. As the result, not only the quality of data transmitted by radiobut also the data transmission efficiency are improved.

According to an eighth aspect of the present invention, in the datatransmission method of the first aspect, in the transmission-sideprocess, the uncompressed packet is transmitted when the size of thecompressed data included in the compressed packet to be transmitted tothe receiving end exceeds a predetermined value. Therefore, the datasize of the compressed packet is minimized, and the compressionefficiency of the transmission data is improved, whereby not only thequality of data transmitted by radio but also the data transmissionefficiency are improved.

According to a ninth aspect of the present invention, in the datatransmission method of the first aspect, in the reception-side process,a request for transmission of the uncompressed packet is output to thetransmitting end when the size of the compressed data included in thecompressed packet supplied from the transmitting end exceeds apredetermined value. In the transmission-side process, on receipt of therequest from the receiving end, the uncompressed packet is transmittedto the receiving end. Therefore, the data size of the compressed packetis minimized, and the compression efficiency of the transmission data isimproved, whereby not only the quality of data transmitted by radio butalso the data transmission efficiency are improved.

According to a tenth aspect of the present invention, in the datatransmission method of the first aspect, in the transmission-sideprocess, the uncompressed packet containing the same transmission datais continuously transmitted by a predetermined number of times to thereceiving end. Therefore, the reliability of the transmission processfor the uncompressed packet required for restoration of the compressedpacket is improved, whereby the quality of data transmitted by radio issignificantly improved.

According to an eleventh aspect of the present invention, in the datatransmission method of the tenth aspect, in the reception-side process,when a restoration error of the compressed data stored in the compressedpacket is detected, this restoration error is notified to thetransmitting end. In the transmission-side process, the number of timesthat the uncompressed packet is transmitted to the receiving end ischanged on the basis of the frequency of notification of restorationerror from the receiving end. Therefore, continuous transmission of theuncompressed packet is performed with efficiency.

According to a twelfth aspect of the present invention, in the datatransmission method of the first aspect, in the transmission-sideprocess, after transmission of the uncompressed packet, an auxiliarytransmission packet including the packet identifier and the transmissiondata stored in the uncompressed packet is transmitted by a predeterminednumber of times to the receiving end. Therefore, the reliability of thetransmission process for the information required for restoration of thecompressed packet is improved, whereby the quality of data transmittedby radio is significantly improved.

According to a thirteenth aspect of the present invention, in the datatransmission method of the first aspect, in the transmission-sideprocess, the uncompressed packet, to which an error correction code isadded, is transmitted to the receiving end, and in the reception-sideprocess, the uncompressed packet is subjected to error correctionaccording to the error correction code. Therefore, the reliability ofthe transmission process for the uncompressed packet which includesinformation required for restoration of the compressed packet isimproved, whereby the quality of data transmitted by radio issignificantly improved.

According to a fourteenth aspect of the present invention, in the datatransmission method of the second aspect, in the transmission-sideprocess, error correction codes are added to the packet identifier andthe transmission data which are stored in the uncompressed packet, andin the reception-side process, the packet identifier and thetransmission data included in the uncompressed packet are subjected toerror correction according to the error correction codes. Therefore, thereliability of the transmission process for the information required forrestoration of the compressed packet is improved, whereby the quality ofdata transmitted by radio is significantly improved.

According to a fifteenth aspect of the present invention, in the datatransmission method of the first aspect, in the reception-side process,when a restoration error of the compressed data stored in the compressedpacket is detected, this restoration error is notified to thetransmitting end. In the transmission-side process, according to thefrequency of notification of restoration error from the receiving end,one of the following two processes is performed: a process oftransmitting the uncompressed packet after attaching an error correctioncode to this packet, and a process of transmitting the uncompressedpacket without attaching an error correction code to this packet.Therefore, assignment of an error correction code to the uncompressedpacket, and error correction for the uncompressed packet are performedeffectively.

According to a sixteenth aspect of the present invention, in the datatransmission method of the first aspect, in the transmission-sideprocess, only the uncompressed packet is stored as data to beretransmitted in a buffer for retransmission. In the reception-sideprocess, when a transmission error of the uncompressed packet isdetected, a request for retransmission of the uncompressed packet as theerror packet is output to the transmitting end. In the transmission-sideprocess, on receipt of the request for retransmission, the uncompressedpacket corresponding to the error packet is retransmitted to thereceiving end only when the uncompressed packet is stored in the buffer.Therefore, the reliability of the transmission process for theinformation required for restoration of the compressed packet isimproved, whereby the quality of data transmitted by radio issignificantly improved.

According to a seventeenth aspect of the present invention, in the datatransmission method of the second aspect, in the transmission-sideprocess, the packet identifier and the transmission data which areincluded in the uncompressed packet are stored as data to beretransmitted in a buffer for retransmission. In the reception-sideprocess, when a transmission error of the uncompressed packet isdetected, a request for retransmission of the packet identifier and thetransmission data stored in the uncompressed packet as the error packetis output to the transmitting end. In the transmission-side process, onreceipt of the request for retransmission, the packet identifier and thetransmission data stored in the uncompressed packet as the error packetare retransmitted to the receiving end only when these are stored in thebuffer. Therefore, the reliability of the transmission process for theinformation required for restoration of the compressed packet isimproved, and retransmission of the uncompressed packet is performedwith efficiency and, furthermore, the data storage capacity of thebuffer for retransmission can be reduced.

According to an eighteenth aspect of the present invention, there isprovided a data transmission method for sequentially transmitting datain units of packets each containing transmission data, from thetransmitting end to the receiving end, and this method comprises a firstdata transmission process and a second data transmission process. Thefirst data transmission process includes a transmission-side process oftransmitting an uncompressed packet in which predetermined transmissiondata is stored as uncompressed data, and then continuously transmittinga compressed packet in which at least a portion of transmission datafollowing the predetermined transmission data is compressed and storedas compressed data; and a reception-side process of receiving thepackets transmitted from the transmitting end, and restoring thetransmission data of the respective packets on the basis of theuncompressed data and the compressed data stored in the respectivepackets. The transmission-side process includes a compression process offorming compressed data to be stored in a compressed packet to betransmitted, on the basis of the transmission data of a reference packetthat is the uncompressed packet, and the transmission data of thecompressed packet to be transmitted. The reception-side process includesa restoration process of restoring the transmission data of a compressedpacket to be restored, on the basis of the transmission data of thereference packet, and the compressed data included in the compressedpacket to be restored. The second data transmission process is forforming, at the transmitting end, compressed data to be stored in thecompressed packet by a formation method different from the compresseddata formation method employed in the first data transmission process,and restoring, at the receiving end, the compressed data stored in thecompressed packet by a restoration. method different from the compresseddata restoration method employed in the first data transmission process.In this method, when transmitting the transmission data in packet units,the data transmission process is switched between the first process andthe second process according to whether a restoration error occurs inthe compressed packet at the receiving end. Therefore, the quality ofdata transmitted by radio is improved when the error frequency is high,and the compression efficiency of transmission data is improved when theerror frequency is low.

According to a nineteenth aspect of the present invention, in the datatransmission method of the eighteenth aspect, the second datatransmission process includes, as a transmission-side process, acompression process of forming compressed data to be stored in acompressed packet to be transmitted, on the basis of the transmissiondata of a previous packet which has been transmitted immediately beforethe compressed packet, and the transmission data of the compressedpacket to be transmitted; and as a reception-side process, a restorationprocess of restoring the compressed data included in a compressed packetto be restored, by using the transmission data of the previous packet.Therefore, when the error frequency is low, the compression efficiencyof transmission data is significantly improved.

According to a twentieth aspect of the present invention, in the datatransmission method of the nineteenth aspect, at the receiving end, whenan error occurs in the restoration process of restoring the compresseddata included in the compressed packet, the receiving end notifies thetransmitting end of this error. At the transmitting end, when thefrequency of error notification exceeds a predetermined value, thetransmitting end requests the receiving end to change the restorationprocess at the receiving end to the restoration process in the firstdata transmission process and, thereafter, the transmitting end performsthe compression process in the first data transmission process. On theother hand, when the frequency of error notification becomes equal to orsmaller than the predetermined value, the transmitting end requests thereceiving end to change the restoration process at the receiving end tothe restoration process in the second transmission process and,thereafter, the transmitting end performs the compression process in thesecond data transmission process. Therefore, it is possible toadaptively switch the transmission method between the transmissionmethod which provides high quality of radio-transmitted data (in thecase where the error frequency is high) and the transmission methodwhich provides high compression efficiency of transmission data (in thecase where the error frequency is low).

According to a twenty-first aspect of the present invention, in the datatransmission method of the nineteenth aspect, at the receiving end, whenthe frequency of error which occurs in the restoration process ofrestoring the compressed data included in the compressed packet exceedsa predetermined value, the receiving end requests the transmitting endto change the compression process at the transmitting end to thecompression process in the first data transmission process. On the otherhand, when the frequency of error in the restoration process becomesequal to or lower than the predetermined value, the receiving endrequests the transmitting end to change the compression process at thetransmitting end to the compression process in the second datatransmission process. The transmitting end performs either thecompression process in the first data transmission process or thecompression process in the second data transmission process, accordingto the request from the receiving end. Therefore, it is possible toadaptively switch the transmission method between the transmissionmethod which provides high quality of radio-transmitted data (in thecase where the error frequency is high) and the transmission methodwhich provides high compression efficiency of transmission data (in thecase where the error frequency is low).

According to a twenty-second aspect of the present invention, there isprovided a data transmission method for sequentially transmitting datain units of packets each containing transmission data, from thetransmitting end to the receiving end. This method comprises atransmission-side process of transmitting an uncompressed packet inwhich predetermined transmission data is stored as uncompressed data,and then continuously transmitting a compressed packet in which at leasta portion of transmission data following the predetermined transmissiondata is compressed and stored as compressed data; and a reception-sideprocess of receiving the packets from the transmitting end, andrestoring the transmission data of the respective packets on the basisof the uncompressed data and the compressed data stored in therespective packets. The transmission-side process includes a compressionprocess of forming compressed data to be stored in a compressed packetto be transmitted, on the basis of updation information relating to apacket which has been transmitted prior to the compressed packet and thetransmission data of the compressed packet to be transmitted; and atransmission-side updation process of setting information relating tothe uncompressed packet as an initial value of the updation information,and updating the updation information to information relating to aspecific compressed packet every time the specific compressed packet isformed. The reception-side process includes a restoration process ofrestoring the transmission data of a compressed packet to be restored byusing updation information relating to a packet which has been receivedprior to the compressed packet; and a reception-side updation process ofsetting information relating to the uncompressed packet as an initialvalue of the updation information and, thereafter, updating the updationinformation to information relating to the specific compressed packetevery time the transmission data of the specific compressed packet isrestored. Therefore, the quality of data transmitted in the radiosection is improved to increase the effective rate of data transmissionand, further, the data compression efficiency is improved. As theresult, the time and cost required for transmission of unrestorablepackets are significantly reduced. Further, since the referenceinformation required for restoration of the compressed packet is updatedby transmission of the specific compressed packet, the compressionefficiency of transmission data is improved while maintaining highefficiency of data transmission.

According to a twenty-third aspect of the present invention, in the datatransmission method of the twenty-second aspect, the updationinformation is composed of a reference packet identifier whichindicates, as a reference packet, either the uncompressed packet or thespecific compressed packet, and the transmission data corresponding tothe reference packet; the compressed packet includes a reference packetidentifier which indicate, as a reference packet, either theuncompressed packet or the specific compressed packet, and aninformation updation flag indicating whether the updation information isto be updated; the information updation flag included in the specificcompressed packet is set at a value indicating that the updationinformation is to be updated; and the information updation flagsincluded in compressed packets other than the specific compressed packetare set at a value indicating that the updation information is not to beupdated. Therefore, the receiving end can easily decide whether theupdation information is to be updated, according to the informationupdation flag.

According to a twenty-fourth aspect of the present invention, in thedata transmission method of the twenty-second aspect, in thetransmission-side process, the specific compressed packet is transmittedto the receiving end every time a predetermined period of time haspassed. Therefore, the quantity of difference data in the compressedpacket is prevented from increasing, whereby the quality of datatransmitted in the radio section is improved to increase the effectiverate of data transmission and, further, the data compression efficiencyis improved.

According to a twenty-fifth aspect of the present invention, in the datatransmission method of the twenty-second aspect, in thetransmission-side process, the specific compressed packet is transmittedto the receiving end every time a predetermined number of compressedpackets have been transmitted. Therefore, the quantity of differencedata in the compressed packet is prevented from increasing, whereby thequality of data transmitted in the radio section is improved to increasethe effective rate of data transmission and, further, the datacompression efficiency is improved.

According to a twenty-sixth aspect of the present invention, in the datatransmission method of the twenty-second aspect, in thetransmission-side process, the specific compressed packet is transmittedto the receiving end when transmission of the specific compressed packetis requested from the receiving end. Therefore, the quality of datatransmitted in the radio section is improved, and the effective rate ofdata transmission is increased. Further, the data compression efficiencyis improved.

According to a twenty-seventh aspect of the present invention, in thedata transmission method of the twenty-second aspect, in thetransmission-side process, the specific compressed packet is transmittedwhen the size of the compressed data included in the compressed packetto be transmitted to the receiving end exceeds a predetermined value.Therefore, the quantity of difference data in the compressed packet isprevented from increasing.

According to a twenty-eighth aspect of the present invention, in thedata transmission method of the twenty-second aspect, in thetransmission-side process, the specific compressed packet is transmittedwhen the average of sizes of the compressed data included in thecompressed packets to be transmitted to the receiving end exceeds apredetermined value. Therefore, the quantity of difference data in thecompressed packet is prevented from increasing, and control againstvariation of the difference data is performed with stability. Also inthis case, the time and cost required for transmission of unrestorablepackets are significantly reduced.

According to a twenty-ninth aspect of the present invention, in the datatransmission method of the twenty-second aspect, the transmission dataincludes plural pieces of item-basis transmission data corresponding todifferent items; the compressed data includes plural pieces ofitem-basis compressed data corresponding different items; the item-basiscompressed data corresponding to each item in the compressed dataincluded in the compressed packet is obtained by compressing theitem-basis compressed data corresponding to each item in thetransmission data of the compressed packet by using the item-basistransmission data corresponding to each item in the transmission data ofthe uncompressed packet or the specific compressed packet; and each ofthe item-basis compressed data includes an item type flag whichspecifies the item corresponding to the compressed data. Therefore, thetransmission data is compressed for each item, whereby optimumcompression effect is realized for each item. Further, the storage area(e.g., RAM) for storing the updated information and the like is reduced.Thereby, the time and cost required for transmission of unrestorablepackets are reduced and, further, the cost for fabrication oftransmission terminal equipment or reception terminal equipment isreduced.

According to a thirtieth aspect of the present invention, in the datatransmission method of the twenty-ninth aspect, each of the item-basiscompressed data includes data length information indicating the lengthof the compressed data. Therefore, the item-basis compressed data isrestored with efficiency.

According to a thirty-first aspect of the present invention, in the datatransmission method of the twenty-ninth aspect, the respectiveitem-basis compressed data are formed using different compressionmethods; and each of the item-basis compressed data includes arestoration method information which indicates a restoration methodcorresponding to the compression method. Therefore, a plurality ofitem-basis compressed data which have been obtained by differentcompression processes are restored with efficiency.

According to a thirty-second aspect of the present invention, there isprovided a data transmission apparatus for sequentially transmittingdata in units of packets each containing transmission data, to thereceiving end. This apparatus comprises a reception unit for receivingthe transmission data as an input signal; a packet formation unit forreceiving the transmission data received, and forming an uncompressedpacket in which predetermined transmission data is stored asuncompressed data, and a compressed packet in which at least a portionof transmission data that follows the predetermined transmission data iscompressed and stored as compressed data; a reference informationmanagement unit for holding and managing, as reference information,information relating to the uncompressed packet formed by the packetformation unit; and a transmission unit for transmitting the respectivepackets formed by the packet formation unit, as a transmission signal,to the receiving end. The packet formation unit forms compressed data tobe stored in a compressed packet to be formed, on the basis of thetransmission data of the uncompressed packet and the referenceinformation stored in the reference information management unit.Therefore, even when a transmission error occurs in the compressedpacket transmitted in the radio section, the receiving end can restorethe subsequent compressed packet with reference to the transmission dataof the uncompressed packet as the reference packet. Thereby, the numberof packets to be discarded at the receiving end due to the transmissionerror in the radio section is reduced, with the result that the qualityof data transmitted in the transmission path including the radio sectionis improved.

According to a thirty-third aspect of the present invention, there isprovided a data reception apparatus for receiving data which have beentransmitted in packet units from the transmitting end as a transmissionsignal, and sequentially restoring transmission data of the respectivepackets. This apparatus comprises a packet reception unit for receivingthe transmission signal, and outputting an uncompressed packet in whichpredetermined transmission data is stored as uncompressed data, and acompressed packet in which at least a portion of transmission datafollowing the predetermined transmission data is compressed and storedas compressed data; a packet restoration unit for receiving the outputfrom the packet reception unit, and restoring the respective packets onthe basis of the data stored in the respective packets, and outputtingthe transmission data of the respective packets; and an output unit foroutputting the transmission data of the respective packets supplied fromthe packet restoration unit. The packet restoration unit restores thetransmission data of the compressed packet, on the basis of thecompressed data included in the compressed packet and the referenceinformation stored in the reference information management unit.Therefore, even when a transmission error occurs in the compressedpacket transmitted in the radio section, the subsequent compressedpacket are restored with reference to the transmission data of theuncompressed packet as the reference packet, whereby the quality of datatransmitted in the transmission path including the radio section isimproved.

According to a thirty-fourth aspect of the present invention, there isprovided a data transmission apparatus for sequentially transmittingdata in units of packets each containing transmission data, to thereceiving end. This apparatus comprises a reception unit for receivingthe transmission data as an input signal; a packet formation unit forreceiving the transmission data, and forming an uncompressed packet inwhich predetermined transmission data is stored as uncompressed data,and a compressed packet in which at least a portion of transmission datafollowing the predetermined transmission data is compressed and storedas compressed data; an information management unit for managing, asupdation information, information relating to the uncompressed packetand a specific compressed packet which are formed by the packetformation unit; and a transmission unit for transmitting the packetsformed by the packet formation unit as a transmission signal to thereceiving end. The information management unit being constructed so thatit sets information relating to the uncompressed packet as an initialvalue of the updation information and, thereafter, updates the updationinformation to the information relating to the specific compressedpacket every time the specific compressed packet is formed. The packetformation unit is constructed so that it forms the compressed data to bestored in a compressed packet to be formed, on the basis of thetransmission data of the compressed packet and the updation informationstored in the reference information management unit. Therefore, thequality of data transmitted in the radio section is improved to increasethe effective rate of data transmission and, moreover, the datacompression efficiency is improved. As the result, the time and costrequired for transmission of unrestorable packets are significantlyreduced. Further, since the reference information required forrestoration of the compressed packet is updated by transmission of thespecific compressed packet, the number of times the uncompressed packetis transmitted can be minimized.

According to a thirty-fifth aspect of the present invention, there isprovided a data reception apparatus for receiving data which have beentransmitted in packet units from the transmitting end as a transmissionsignal, and sequentially restoring transmission data of the respectivepackets. This apparatus comprises a packet reception unit for receivingthe transmission signal, and outputting an uncompressed packet in whichpredetermined transmission data is stored as uncompressed data, and acompressed packet in which at least a portion of transmission datafollowing the predetermined transmission data is compressed and storedas compressed data; a packet restoration unit for receiving the outputfrom the packet reception unit, and restoring the respective packets onthe basis of the data stored in the respective packets, and outputtingthe transmission data of the respective packets; a reference informationmanagement unit for storing and managing, as reference information,information relating to the uncompressed packet and the specificcompressed packet which are restored by the packet restoration unit; andan output unit for outputting the transmission data of the respectivepackets which are restored by the packet restoration unit. Theinformation management unit is constructed so that it sets theinformation relating to the uncompressed packet as an initial value ofthe updation information and, thereafter, updates the updationinformation to the information relating to the specific compressedpacket every time the specific compressed packet is restored. The packetrestoration unit is constructed so that it restores the transmissiondata of the compressed packet, on the basis of the compressed dataincluded in the compressed packet and the reference information storedin the reference information management unit. Therefore, the quality ofdata transmission in the radio section is improved to increase theeffective rate of data transmission, and the data compression efficiencyis improved. As the result, the time and cost required for transmissionof unrestorable packets are significantly reduced. Further, since thereference information required for restoration of the compressed packetis updated every time the specific compressed packet is restored, it ispossible to realize data transmission with high data transmissionefficiency and high data compression efficiency while minimizing thenumber of times the uncompressed packet is transmitted.

According to a thirty-sixth aspect of the present invention, there isprovided a data structure of a compressed packet which includescompressed data obtained by compressing at least a portion oftransmission data and is to be transmitted after a reference packetwhich is used for restoration of the compressed data. The compressedpacket comprises a data section in which the compressed data is stored,and a header section including a first identifier which indicateswhether the data stored in the data section is compressed, and a secondidentifier which identifies the reference packet. Therefore, even when atransmission error occurs in the compressed packet transmitted in theradio section, the receiving end can restore the subsequent compressedpackets with reference to the transmission data of the uncompressedpacket as the reference packet.

According to a thirty-seventh aspect of the present invention, in thepacket data structure of the thirty-sixth aspect, the transmission datacomprises plural pieces of item-basis compression target datacorresponding to difference items to be compressed, and non-target datawhich is not to be compressed; the data section of the compressed packetincludes, as the compressed data, item-basis compressed datacorresponding to the respective items, and the non-target data; theitem-basis compressed data corresponding to each item is restorable onthe basis of item-basis uncompressed data corresponding to each item andstored in the reference packet; and the header section of the compressedpacket includes additional information for calculating the item-basiscompressed data corresponding to a predetermined target item, on thebasis of the corresponding item-basis uncompressed data in the referencepacket. Therefore, the transmission data is compressed for each item,whereby optimum compression effect is realized for each item. Further,the storage area (e.g., RAM) for storing the information relating to thereference packet and the like is reduced. Thereby, the time and costrequired for transmission of unrestorable packets are reduced and,further, the cost for fabrication of transmission terminal equipment orreception terminal equipment is reduced.

According to a thirty-eighth aspect of the present invention, there isprovided a data structure of a compressed packet which includescompressed data obtained by compressing at least a portion oftransmission data and is to be transmitted after a reference packetwhich is used for restoration of the compressed data. The compressedpacket comprises a data section in which the compressed data is stored,and a header section including a first identifier which indicateswhether the data stored in the data section is compressed, a secondidentifier which identifies the reference packet, and a referenceinformation updation flag which indicates whether reference informationcorresponding to the transmission data of the reference packet is to beupdated. Therefore, the quality of data transmitted in the radio sectionis improved to increase the effective rate of data transmission and,further, the data compression efficiency is improved. As the result, thetime and cost required for transmission of unrestorable packets aresignificantly reduced. Further, since the reference information requiredfor restoration of the compressed packet, the number of times theuncompressed packet is transmitted can be minimized.

According to a thirty-ninth aspect of the present invention, in thepacket data structure of the thirty-eighth aspect, the transmission datacomprises plural pieces of item-basis compression target datacorresponding to difference items to be compressed, and non-target datawhich is not to be compressed; the data section of the compressed packetincludes, as the compressed data, item-basis compressed datacorresponding to the respective items, and the non-target data; theitem-basis compressed data corresponding to each item is restorable onthe basis of item-basis uncompressed data corresponding to each item andstored in the reference packet; and the header section of the compressedpacket includes additional information for calculating the item-basiscompressed data corresponding to a predetermined target item, on thebasis of the corresponding item-basis uncompressed data in the referencepacket. Therefore, the transmission data can be compressed for eachitem, whereby optimum compression effect is realized for each item.Further, the storage area (e.g., RAM) for storing the informationrelating to the reference packet and the like is reduced. Thereby, thetime and cost required for transmission of unrestorable packets arereduced and, furthermore, the cost for fabrication of transmissionterminal equipment or reception terminal equipment is reduced.

According to a fortieth aspect of the present invention, in the packetdata structure of the thirty-ninth aspect, the header section of thecompressed packet includes a data existence flag which indicates whetheror not any of the plural item-basis compressed data is included in thedata section of the compressed packet. Therefore, it is easily decidedin short time whether the compressed packet is to be restored.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are diagrams for explaining a data transmissionmethod according to a first embodiment of the present invention,illustrating data structures of an uncompressed packet (1(a)) and acompressed packet (1(b)) which are used in the data transmission method.

FIG. 2 is a block diagram for explaining a data transmission systemusing the data transmission method of the first embodiment, illustratinga data transmission apparatus in the data transmission system.

FIG. 3 is a block diagram for explaining a data transmission systemusing the data transmission method of the first embodiment, illustratinga data reception apparatus in the data transmission system.

FIG. 4 is a diagram for explaining the data transmission method of thefirst embodiment, illustrating the flow of plural packets from thetransmitting end to the receiving end in the normal transmission state.

FIG. 5 is a diagram for explaining the data transmission method of thefirst embodiment, illustrating the flow of plural packets from thetransmitting end to the receiving end in the state where a transmissionerror occurs.

FIG. 6 is a flowchart for explaining packet formation by the datatransmission apparatus of the first embodiment.

FIG. 7 is a flowchart for explaining packet restoration by the datareception apparatus of the first embodiment.

FIGS. 8(a) and 8(b) are diagrams for explaining a data transmissionmethod according to a first modification of the first embodiment,illustrating the data structures of an uncompressed packet (8(a)) and acompressed packet (8(b)) which are used in the data transmission method.

FIG. 9 is a diagram for explaining the data transmission methodaccording to the first modification of the first embodiment,illustrating the flow of plural packets from the transmitting end to thereceiving end in the normal transmission state.

FIGS. 10(a) and 10(b) are diagrams for explaining a data transmissionmethod according to a second modification of the first embodiment,illustrating the data structures of an uncompressed packet (10(a)) and acompressed packet (10(b)) which are used in the data transmissionmethod.

FIGS. 11(a) and 11(b) are diagrams for explaining a data transmissionmethod according to a third modification of the first embodiment,illustrating the data structures of an uncompressed packet (11(a)) and acompressed packet (11(b)) which are used in the data transmissionmethod.

FIG. 12 is a block diagram for explaining a data transmission systemusing a data transmission method according to a second embodiment of thepresent invention, illustrating a data transmission apparatus in thedata transmission system.

FIG. 13 is a diagram for explaining the data transmission method of thesecond embodiment, illustrating the flow of plural packets from thetransmitting end to the receiving end in the normal transmission state.

FIG. 14 is a block diagram for explaining a data transmission systemusing a data transmission method according to a third embodiment of thepresent invention, illustrating a data transmission apparatus in thedata transmission system.

FIG. 15 is a block diagram for explaining the data transmission systemusing the data transmission method of the third embodiment, illustratinga data reception apparatus in the data transmission system.

FIG. 16 is a block diagram for explaining a data transmission systemusing a data transmission method according to a fourth embodiment of thepresent invention, illustrating a data transmission apparatus in thedata transmission system.

FIG. 17 is a block diagram for explaining the data transmission systemusing the data transmission method of the fourth embodiment,illustrating a data reception apparatus in the data transmission system.

FIGS. 18(a) and 18(b) are diagrams for explaining a data transmissionmethod according to a fifth embodiment of the present invention,illustrating the data structures of an uncompressed packet (18(a)) and acompressed packet (18(b)) which are used in the data transmissionmethod.

FIG. 19 is a block diagram for explaining a data transmission systemusing the data transmission method of the fifth embodiment, illustratinga data transmission apparatus in the data transmission system.

FIG. 20 is a block diagram for explaining the data transmission systemusing the data transmission method of the fifth embodiment, illustratinga data reception apparatus in the data transmission system.

FIG. 21 is a diagram for explaining the data transmission method of thefifth embodiment, illustrating the flow of plural packets from thetransmitting end to the receiving end in the normal transmission state.

FIG. 22 is a diagram for explaining the data transmission method of thefifth embodiment, illustrating the flow of plural packets from thetransmitting end to the receiving end in the state where a transmissionerror occurs.

FIG. 23 is a flowchart for explaining packet formation by the datatransmission apparatus of the fifth embodiment.

FIG. 24 is a flowchart for explaining packet restoration by the datareception apparatus of the fifth embodiment.

FIGS. 25(a)-25(c) are diagrams for explaining a data transmission methodaccording to a modification of the fifth embodiment, illustrating thedata structures of an uncompressed packet (25(a)) and a compressedpacket (25(b)) which are used in the data transmission method, and aprocess of forming a compressed packet Pj(Y) by compressing transmissiondata D(Y) (25(c)).

FIG. 26 is a diagram illustrating a part of the compressed packet Pj(Y)according to the modification of the fifth embodiment.

FIGS. 27(a), 27(b) and 27(c) are diagrams for explaining transmissiondata to be transmitted (27(a)) and specific data stored in anuncompressed packet Pi and a compressed packet Pj (27(b)), according tothe modification of the fifth embodiment.

FIGS. 28(a) and 28(b) are diagrams for explaining a data transmissionsystem to which the conventional header compression method by V.Jacobson is applied, illustrating the whole structure of the datatransmission system (28(a)) and processes required for data transmissionin the data transmission system (28(b)).

FIGS. 29(a)-29(e) are diagrams illustrating the data structures ofpackets used in the conventional data transmission system and, moreparticularly, illustrating an RTP packet (29(a)), an UDP packet (29(b)),IP packets (29(c),29(d)), and a PPP packet (29(e)).

FIGS. 30(a) and 30(b) are diagrams illustrating the data structures ofPPP packets used in the data transmission system to which the V.Jacobson's header compression method is applied and, more particularly,illustrating an compressed packet (30(a)) and a compressed packet(30(b)).

FIG. 31 is a diagram for conceptually explaining PPP packet transmissionusing the V. Jacobson's header compression method.

FIG. 32 is a diagram for explaining the case where a transmission erroroccurs in the PPP packet transmission using the V. Jacobson's headercompression method.

FIGS. 33(a) and 33(b) are diagrams for explaining a data transmissionsystem having a radio transmission section to which the V. Jacobson'sheader compression method is applied, illustrating the whole structureof the data transmission system (33(a)) and processes required for datatransmission in the data transmission system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the inventor's viewpoint and the fundamental principle ofthe present invention will be described.

The inventors of the present invention have earnestly studied a methodfor improving the quality of data transmitted through a networkincluding a radio transmission path, and finally discovered that thedata quality can be improved by using, instead of the existing headercompression method (e.g., V. Jacobson's header compression method), aheader compression method in which difference data obtained by usingtransmission data of an uncompressed packet that has been transmittedpreviously to a compressed packet to be transmitted, is stored in thiscompressed packet as compressed data to be transmitted.

In the following description for the embodiments of the presentinvention, as for data communication, only single-direction transmissionfrom a server Sin on the Internet to a mobile radio terminal (e.g.,visual terminal) Tmo will be described as shown by data flow Df in FIG.33. However, by constructing the communication apparatuses such as theserver and the terminal unit for data communication so as to have bothfunctions as a transmitter and a receiver, bi-directional andsimultaneous data communication is realized. In the above-described datacommunication from the server Sin on the Internet to the mobile radioterminal Tmo, the gateway server Sga (refer to FIG. 33(b)) performsreception of data from the server Sin as well as transmission of thereceived data to the terminal Tmo, and the mobile radio terminal Tmoperforms reception of the data from the gateway server Sga.

Further, the embodiments of the present invention will be described forthe case where an RTP/UDP/IP packet corresponding to an IP packet Pipb(refer to FIG. 29(d)) is transmitted as data to be transmitted(transmission data) including various kinds of header information, withthe header information being compressed, by using PPP. However, thetransmission data and the transmission protocol are not restricted tothe RTP/UDP/IP packet and the PPP.

[Embodiment 1]

FIGS. 1 to 11 are diagrams for explaining a data transmission methodaccording to a first embodiment of the present invention. This firstembodiment corresponds to aspects 1˜9, 18˜21, 32, 33, 36, and 37.

In the data transmission method of this first embodiment, datatransmission from a transmitter to a receiver is performed packet bypacket. The transmitter forms uncompressed packets and compressedpackets and transmits these packets, and the receiver receives thesepackets from the transmitter and sequentially restores the receivedpackets. In this method, difference data, which is based on transmissiondata stored in an uncompressed packet that has been transmittedmost-recently, is stored in a compressed packet to be transmitted. To bespecific, the difference data stored in the compressed packet to betransmitted is obtained by using the transmission data stored in theuncompressed packet as reference data, and subtracting the transmissiondata to be transmitted by the compressed packet from the reference data.

In the following description, transmission data stored in anuncompressed packet is also referred to as “transmission data of anuncompressed packet”, and transmission data to be transmitted by acompressed packet is also referred to as “transmission data of acompressed packet”.

FIGS. 1(a) and 1(b) are diagrams for explaining the data transmissionmethod according to the first embodiment, illustrating the datastructures of an uncompressed packet (FIG. 1(a)) and a compressed packet(FIG. 1(b)) which are employed in the data transmission method. In FIGS.1(a) and 1(b), only portions of these PPP packets, required forexplaining the header compression method, are shown in detail.

As shown in FIG. 1(a), an uncompressed packet Pa is composed of a headersection Hpa containing header information, and a data section Dpacontaining uncompressed data Ir to be transmitted by PPP. Theinformation in the header section Hpa is composed of acompression/uncompression identifier Ih1 indicating whether the datastored in the data section Dpa is compressed, a packet identifier (ID)Ih2 a for identifying this packet, and other header information Ih3. Theidentifier Ih1 of this uncompressed packet Pa indicates “uncompressed”.The uncompressed data Ir is transmission data (D) to be transmitted bythe uncompressed packet.

On the other hand, as shown in FIG. 1(b), the compressed packet Pb iscomposed of a header section Hpb containing header information, and adata section Dpb containing compressed data Id to be transmitted by PPP.The information in the header section Hpb is composed of acompression/uncompression identifier Ih1 indicating whether the datastored in the data section Dpb is compressed, a reference packetidentifier (ID) Ih2 b for identifying an uncompressed packet (referencepacket) which contains transmission data to be used as reference data,and other header information Ih3. The identifier Ih1 of the compressedpacket Pb indicates “compressed”. The compressed data Id is differencedata (ΔD) between the transmission data (reference data) of amost-recent uncompressed packet (reference packet) which has beentransmitted previously to the compressed packet Pb, and the transmissiondata of the compressed packet Pb.

It is needless to say that the header information Ih3 includes a CRCcode Icrc shown in FIG. 29(e).

FIG. 2 is a block diagram for explaining a data transmission apparatus101 in a data transmission system which performs data transmissionaccording to the data transmission method of this first embodiment.

The data transmission apparatus 101 corresponds to the gateway serverSga in the data transmission system Cs2 shown in FIG. 33(a). The datatransmission apparatus 101 includes a reception unit 11, acompressed/uncompressed packet formation unit 12, and a packettransmission unit 16. The reception unit 11 receives a firsttransmission signal S1 including transmission data, supplied from theInternet In to the receiving end (mobile radio terminal Tmo), andoutputs the transmission data as a reception signal Src. The packetformation unit 12 packetizes the transmission data from the receptionunit 11 on the basis of transmission standard such as PPP, and outputsan uncompressed packet Pa or a compressed packet Pb. The packettransmission unit 16 transmits the packet formed by the unit 12 as asecond transmission signal S2 to the receiving end, by a transmissionmethod such as W-CDMA.

Further, the data transmission apparatus 101 includes an errornotification reception unit 14 and a compression/uncompression decisionunit 13. The error notification reception unit 14 receives a restorationerror notification signal Ne from the receiving end, which indicatesthat a restoration error has occurred at the receiving end, and outputsan error notification reception signal Sn. The decision unit 13 managesthe type of each packet formed by the packet formation unit 12, decidesthe type of a packet to be formed next on the basis of the managedpacket type and the error notification reception signal Sn, and outputsa packet decision signal Jp. In the decision unit 13, thecompression/uncompression identifiers Ih1 of the packets formed by thepacket formation unit 12 are recorded in association with thecorresponding packets. Further, in the decision unit 13, the packet typeis decided as follows. A packet which is formed first after startingdata transmission, and a packet which is formed immediately afterreceiving an error notification reception signal Sn are decided as“uncompressed packets”, while the other packets are decided as“compressed packets”.

Further, the data transmission apparatus 101 includes a referenceinformation management unit 15. This management unit 15 associates thetransmission data (D) stored as uncompressed data Ir in the uncompressedpacket Pa with the packet identifier ID) Ih2 a for identifying thisuncompressed packet, and manages them as transmitting-end referenceinformation Im1. The transmitting-end reference information Im1 iscomposed of an identifier (ID) equal to the packet identifier (ID) Ih2a, and reference data (D) equal to the transmission data (D) of theuncompressed packet Pa. This management unit 15 updates the packetidentifier (ID) and the reference data (D) which are stored as thetransmitting-end reference information Im1, every time an uncompressedpacket is formed by the packet formation unit 12, according to atransmitting-end management control signal Cm1 supplied from theformation unit 12.

In the data transmission apparatus 101, the compressed/uncompressedpacket formation unit 12 forms either an uncompressed packet Pa or acompressed packet Pb on the basis of the packet decision signal Jp. Whenforming the uncompressed packet Pa, the transmission data (D) is storedas the uncompressed data Ir in its data section Dpa, and the (ID) whichspecifies the uncompressed packet Pa is stored as the packet identifierIh2 a in its header section Hpa. When forming the compressed packet Pb,the difference data (ΔD) based on the transmission data (D) of theuncompressed packet (reference packet) is stored as the compressed dataId in its data section Dpb, and the (ID) which identifies theuncompressed packet Pa (reference packet) is stored as the referencepacket identifier Ih2 b in its header section Hpb.

The difference data (ΔD) of each compressed packet Pb is a differencebetween the transmission data of the compressed packet and thetransmission data of the uncompressed packet as the reference packet.The uncompressed packet as the reference packet is a most-recent packetamongst the uncompressed packets which have been formed previously tothe compressed packet Pb.

FIG. 3 is a block diagram for explaining a data reception apparatus 201in the data transmission system performing data transmission accordingto the data transmission method of this first embodiment.

The data reception apparatus 201 corresponds to the mobile radioterminal Tmo in the data transmission system Cs2 shown in FIG. 33(a).

The data reception apparatus 201 includes a packet reception unit 21 andan error packet detection unit 22. The packet reception unit 21 receivesthe packet which has been transmitted from the transmitting end as thesecond transmission signal S2, by a method such as W-CDMA. The errorpacket detection unit 22 receives the packet Rp output from thereception unit 21, detects an error packet in which an error hasoccurred during transmission or outputs the packet Rp which has beennormally transmitted, as a normal packet Pno.

Further, the data reception apparatus 201 includes a packet restorationunit 23, an error notification transmission unit 24, and an output unit26. The packet restoration unit 23 receives the normal packet Pno fromthe detection unit 22, and restores this packet Pno to output restoreddata Irs. Further, the restoration unit 23 outputs an error signal Sewhen a restoration error occurs. The error notification transmissionunit 24 receives the error signal Se, and outputs a restoration errornotification signal Ne which indicates that the restoration error hasoccurred, to the transmitting end. The output unit 26 outputs thetransmission data (D) which is the restored data Irs, as an outputsignal S3.

In the packet restoration unit 23, as a restoration process for theuncompressed packet Pa, the transmission data (D) is taken from the datasection Dpa of the uncompressed packet Pa on the basis of PPP or thelike. Further, as a restoration process for the compressed packet Pb,the difference data Id is taken from the data section Dpb of thecompressed packet Pb on the basis of PPP or the like, and thetransmission data of this compressed packet is restored with referenceto the transmission data of the uncompressed packet as the referencepacket.

Further, the data reception apparatus 201 includes a referenceinformation management unit 25. When the uncompressed packet Pa isrestored in the restoration unit 23, the management unit 25 associatesthe packet identifier (ID) Ih2 a of the restored uncompressed packet Pawith the transmission data (D) of this uncompressed packet Pa, andmanages them as receiving-end reference information Im2. Thereceiving-end reference information Im2 is composed of an identifier(ID) equal to the packet identifier (ID) Ih2 a, and reference data (D)equal to the transmission data (D) of the uncompressed packet Pa. Themanagement unit 25 updates the packet identifier (ID) and the referencedata (D) which are stored as the receiving-end reference informationIm2, every time an uncompressed packet is restored in the packetrestoration unit 23, according to a receiving-end management controlsignal Cm2 supplied from the restoration unit 23.

In the data reception apparatus 201, when each compressed packet isrestored by the packet restoration unit 23, the reference packetidentifier (ID) stored in this compressed packet and the transmissiondata (D) of this compressed packet are collated with the identifier (ID)and the corresponding reference data (D) which are stored in themanagement unit 25, respectively. Based on the collation, excepting thecase where these packet identifiers (ID) match and these data (D) match,i.e., when either the identifier (ID) or the transmission data (D) ofthe reference packet which is required for restoration of the compressedpacket to be restored is not stored in the management unit 25, an errorsignal Se indicating that a restoration error has occurred is outputfrom the packet restoration unit 23 to the error notificationtransmission unit 24.

Next, the function and effect will be described.

FIGS. 4 and 5 are diagrams for explaining the data transmission methodaccording to the first embodiment. FIG. 4 shows the flow of pluralpackets from the transmitting end to the receiving end in the normaltransmission state, and FIG. 5 shows the flow of plural packets from thetransmitting end to the receiving end when a transmission error occurs.

In FIG. 4, transmission data (D1)˜(D4) are packetized data forpacket-by-packet transmission. In this first embodiment, thetransmission data (D1) is not compressed and transmitted by anuncompressed packet pa(1), and the transmission data (D2)˜(D4) arecompressed and sequentially transmitted by compressed packetsPb(2)˜Pb(4) which follow the uncompressed packet Pa(1). At thetransmitting end, initially, the uncompressed packet Pa(1) is generatedand transmitted to the receiving end. At this time, the transmissiondata (D1) is stored as the uncompressed data Ir in the data section Dpaof the uncompressed packet Pa(1), and the identifier Ih1 indicating“uncompressed”, the packet identifier (ID=0) Ih2 a for identifying thispacket, and other header information rh3 are stored in the headersection Hpa.

Next, the compressed packet Pb(2) is generated and transmitted to thereceiving end. At this time, difference data (D1−D2) is stored as thecompressed data Id in the data section Dpb of this compressed packetPb(2), and the identifier Ih1 indicating “compressed”, the referencepacket identifier (ID=0) Ih2 b, and other header information Ih3 arestored in the header section Hpb. The difference data (D1−D2) isobtained by subtracting the transmission data (D2) of the compressedpacket Pb(2) from the transmission data (D1) of the uncompressed packetPa(1) as the reference packet.

Subsequently, the compressed packet Pb(3) is generated and transmittedto the receiving end. At this time, difference data (D1−D3) is stored asthe compressed data Id in the data section Dpb of this compressed packetPb(3), and the identifier Ih1 indicating “compressed”, the referencepacket identifier (ID=0) Ih2 b, and other header information Ih3 arestored in the header section Hpb. The difference data (D1−D3) isobtained by subtracting the transmission data (D3) of the compressedpacket Pb(3) from the transmission data (D1) of the uncompressed packetPa(1) as the reference packet.

Further, the compressed packet Pb(4) is generated and transmitted to thereceiving end. At this time, difference data (D1−D4) is stored in thedata section Dpb of this compressed packet Pb(4), and the identifier Ih1indicating “compressed”, the reference packet identifier (ID=0) Ih2 b,and other header information Ih3 are stored in the header section Hpb.The difference data (D1−D4) is obtained by subtracting the transmissiondata (D4) of the compressed packet Pb(4) from the transmission data (D1)of the uncompressed packet Pa(1) as the reference packet.

The reference packet identifier (ID=0) Ih2 b which is stored in theheader section Hpb of each of the compressed packets Pb(2), Pb(3), andPb(4) indicates that the reference packet required for restoration ofthis compressed packet is the uncompressed packet Pa(1).

As described above, the uncompressed packet Pa(1) and the followingcompressed packets Pa(2)˜Pa(4) are sequentially received at thereceiving end in the normal transmission state, and the transmissiondata (D1)˜(D4) of these packets are restored.

That is, at the receiving end, when the uncompressed packet Pa(1) isreceived, the transmission data (D1) is taken from the data section Dpa.Subsequently, when the compressed packet Pb(2) is received, thedifference data (D1−D2) is taken from the data section Dpb, and thetransmission data (D2) of the compressed packet Pb(2) is restored fromthe difference data (D1−D2) with reference to the transmission data (D1)of the uncompressed packet Pa(1) which is specified as the referencepacket by the reference packet identifier (ID=0) Ih2 b.

Thereafter, when the compressed packets Pb(3) and Pb(4) are received, asin the case of the compressed packet Pb(2), the difference data (D1−D3)and (D1−D4) are taken from their data sections Dpb, and the transmissiondata (D3) and (D4) of the compressed packets Pb(3) and Pb(4) arerestored from the difference data (D1−D3) and (D1−D4) with reference tothe transmission data (D1) of the uncompressed packet Pa(1) which isspecified as the reference packet by the reference packet identifier(ID=0) Ih2 b.

FIG. 5 shows the case where a transmission error occurs in thecompressed packet Pb(2) during the above-described packet transmission.Even in this case, when the compressed packets Pb(3) and Pb(4) whichfollow the compressed packet Pb(2) are received, the transmission data(D3) and (D4) of these compressed packets Pb(3) and Pb(4) are normallyrestored.

That is, in this first embodiment, the difference data (ΔD) stored inthe data section Dpb of each compressed packet Pb is not the differencedata between the transmission data of the compressed packet Pb and thetransmission data of the packet which has been transmitted just beforethe compressed packet Pb, but the difference data between thetransmission data of the compressed packet Pb and the transmission dataof the most-recent uncompressed packet Pa which has been transmittedpreviously to the compressed packet Pb. Therefore, in the datatransmission system of this first embodiment, even when a transmissionerror has occurred in some compressed packet, this transmission errordoes not adversely affect restoration of a compressed packet which isnormally received after the error packet. Accordingly, when suchtransmission error has occurred, only the error packet is discarded, andno restoration error is notified from the receiving end to thetransmitting end.

When a transmission error has occurred in the uncompressed packet Pa(1)during the packet-by-packet data transmission, a notification signal Neindicating that the transmission error has occurred is transmitted fromthe receiving end to the transmitting end, in˜the same manner asdescribed with respect to FIG. 32. On receipt of this notificationsignal Ne, the transmitting end transmits an uncompressed packet to thereceiving end and, thereafter, compressed packets are sequentiallytransmitted. At the receiving end, the error packet and the subsequentcompressed packets, i.e., those packets which have been received fromwhen the transmission error occurred to when the uncompressed packet isnormally received, are discarded.

Next, the operation of the data transmission apparatus 101 in the datatransmission system will be described.

In the data transmission apparatus 101, as shown in FIG. 4 or 5, thetransmission data (D1)˜(D4) are sequentially transmitted to thereceiving end by the corresponding uncompressed packet and compressedpackets.

For example, when the transmission data (D1)˜(D4) (refer to FIG. 4)which has been transmitted from the server Sin on the Internet (refer toFIG. 33(a)) by a transmission method such as the Ethernet, are input tothe data transmission apparatus 101 as a first transmission signal S1,the reception unit 11 receives these transmission data (D1)˜(D4) by theabove-described transmission method. The received transmission data(D1)˜(D4) are sequentially output to the packet formation unit 12 as areception signal Src.

In the packet formation unit 12, a packet for transmitting eachtransmission data to the receiving end is formed on the basis of atransmission protocol such as PPP. At this time, the packet formationunit 12 inquires of the compression/uncompression decision unit 13 aboutthe type of a packet to be formed. On receipt of this inquiry, thedecision unit 13 provides the packet formation unit 12 with a packetdecision signal Jp indicating the packet type.

To be specific, when the transmission data (D1) is input to the packetformation unit 12, since this is the time to form the first packet afterstarting communication, the decision unit 13 outputs, as a packetdecision signal Jp, information indicating that an uncompressed packetis to be formed, to the packet formation unit 12.

Then, in the formation unit 12, formation of an uncompressed packet isdecided on the basis of the packet decision signal Jp, and anuncompressed packet Pa(1) in which the transmission data (D1) is storedas the uncompressed data Ir is formed.

At this time, “uncompressed” is set as the compression/uncompressionidentifier Ih1 of the uncompressed packet Pa(1), and the identifier(ID=0) which indicates the uncompressed packet Pa(1) is set as thepacket identifier Ih2 a. Further, when the uncompressed packet Pa(1) hasbeen formed in the formation unit 12, the reference informationmanagement unit 15 sets the reference packet identifier (ID) and thereference data (D) as the transmitting-end reference information Im1, tothe identifier (ID=0) indicating the uncompressed packet Pa(1) and thetransmission data (D1) of the uncompressed packet Pa(1), respectively,according to a transmitting end management control signal Cm1 suppliedfrom the formation unit 12.

Then, the uncompressed packet Pa(1) is output to the packet transmissionunit 16, and transmitted to the receiving end by a predetermined radiocommunication method such as W-CDMA.

When the transmission data (D2) output from the reception unit 11 isinput to the packet formation unit 12, since this is not the time toform the first packet after starting communication nor the time to formthe first packet after reception of a restoration error notificationsignal from the receiving end, the decision unit 13 outputs, as a packetdecision signal Jp, information indicating that a compressed packet isto be formed, to the packet formation unit 12.

Then, the formation unit 12 inquires of the management unit 15 about thetransmitting-end reference information Im1. In this case, in themanagement unit 15, the reference packet identifier(ID) is set at theidentifier (ID=0) and the reference data (D) is set at the transmissiondata (D1). Therefore, in the packet formation unit 12, the transmissiondata (D2) is compressed using, as reference data, the transmission data(D1) of the uncompressed packet Pa(1) indicated by the identifier(ID=0). Thereby, difference data (D1−D2) between the transmission data(D1) and the transmission data (D2) is generated as compressed data Idto be stored in the compressed packet.

Subsequently, the packet formation unit 12 forms a compressed packetPb(2) in which the difference data (D1−D2) is stored as compressed dataId of the transmission data (D2). In this compressed packet Pb(2),“compressed” is set as the compression/uncompression identifier Ih1, andthe identifier (ID=0) which indicates the uncompressed packet Pa(1) as areference packet required for restoration of this compressed packetPb(2) is set as the reference packet identifier Ih2 b. Further, when thecompressed packet Pb(2) has been formed in the formation unit 12, thereference information management unit 15 does not update the referencepacket identifier (ID) and the reference data (D) as thetransmitting-end reference information Im1.

This compressed packet Pb(2) is output to the packet transmission unit16, and transmitted to the receiving end (mobile radio terminal) by apredetermined radio communication method such as W-CDMA.

When the transmission data (D3) and (D4) output from the reception unit11 are input to the packet formation unit 12, a compressed packet Pb(3)containing difference data (D1−D3) and a compressed packet Pb(4)containing difference data (D1−D4) are formed, respectively, in the samemanner as described for the transmission data (D2).

FIG. 6 is a flowchart for explaining the procedure performed by thepacket formation unit 12.

When the transmission data (D) received by the reception unit 11 isinput to the packet formation unit 12 (step Sa1), the formation unit 12inquires of the decision unit 13 as to whether a packet to be formed isan uncompressed packet or a compressed packet (step Sa2), and the typeof a packet to be formed is decided on the basis of a packet decisionsignal Jp supplied from the decision unit 13 (step Sa3).

When an uncompressed packet is to be formed, the identifier (ID) isassigned as the packet identifier Ih2 a to the uncompressed packet, andthe uncompressed packet Pa including the packet identifier Ih2 a isformed (step Sa7). Thereafter, according to an instruction from thepacket formation unit 12 (transmitting-end management control signalCm1), the transmitting-end reference information Im1 (i.e., theidentifier (ID) and the reference data (D)) which is stored in thereference information management unit 15 is updated (step Sa8).

On the other hand, when a compressed packet is to be formed, the packetformation unit 12 inquires of the reference information management unit15 as to whether the identifier (ID) and the reference data (D) arestored as the transmitting-end reference information Im1 in themanagement unit 15 (step Sa4). Then, the data section Dpb of thecompressed packet Pb is formed on the basis of the transmitting-sidereference information Iml (i.e., the identifier (ID) and the referencedata (D)) supplied from the management unit 15 (step Sa5). Further, theidentifier (ID) is stored as the reference packet identifier Ih2 b inthe header section Hpb, together with other header information Ih3,whereby the compressed packet Pb is completed (step Sa6).

Then, the uncompressed packet Pa or compressed packet Pb so formed istransmitted to the transmission unit 16 (step Sa9). Thereafter, theformation unit 12 returns to the process of step Sa2.

The above-described process steps in the formation unit 12 are continueduntil transmission of the last transmission data is completed.

Next, the operation of the data reception apparatus 201 in the datatransmission system will be described.

In the data reception apparatus 201, the uncompressed packet and thecompressed packets transmitted from the transmitting end as shown inFIGS. 4 and 5 are sequentially received, and restoration for each packetis performed.

To be specific, in the data reception apparatus 201, the packetreception unit 21 sequentially receives the uncompressed packet Pa(1)and the compressed packets Pb(2)˜Pb(4) which have been transmitted fromthe transmitting end, and the received packets Rp are sequentially inputto the error packet detection unit 22. In the error packet detectionunit 22, each received packet Rp is subjected to error detection. Whenit is conformed that the received packet Rp has been normallytransmitted, this packet Rp is output as a normal packet Pno to thepacket restoration unit 23. On the other hand, when it is not conformedthat the received packet Rp has been normally transmitted, this packetRp is discarded. Although this first embodiment employs CRC (CyclicRedundancy Check) as an error detection method, the error detectionmethod is not restricted thereto.

In the restoration unit 23, when the uncompressed packet Pa(1) is inputas a normal packet Pno, it is detected whether the normal packet Pno isa compressed packet or an uncompressed packet with reference to thecompression/uncompressed identifier Ih1 included in the header sectionof the normal packet Pno. In this case, since the normal packet Pno isthe uncompressed packet Pa(1), the restoration unit 23 takes thetransmission data (D1) from the data section Dpa of the uncompressedpacket Pa(1).

Next, the receiving-end reference information Im2 (i.e., the identifier(ID) and the reference data (D)) which is stored in the management unit25 is updated according to an instruction from the restoration unit 23(receiving-end management control signal cm2). Thereby, the identifier(ID) and the reference data (D) stored in the management unit 25 areupdated to the identifier (ID=0) and the transmission data (D1),respectively. Thereafter, the restoration unit 23 sends the transmissiondata (D1) as the restored data Irs to the output unit 26, and the outputunit 26 outputs the transmission data (D1).

Next, in the restoration unit 23, when the compressed packet Pb(2) isinput as a normal packet Pno, it is detected whether the normal packetPno is a compressed packet or an uncompressed packet with reference tothe compression/uncompression identifier Ih1 included in the headersection of the normal packet Pno. In this case, since the normal packetPno is the compressed packet Pb(2), the restoration unit 23 inquires ofthe reference information management unit 25 as to whether theidentifier (ID=0) which is included in this compressed packet as thereference packet identifier Ih2 b, and the corresponding reference data(D1) are stored in the management unit 25.

In this case, since the identifier (ID=0) and the correspondingreference data (D1) are stored in the management unit 25, therestoration unit 23 restores the transmission unit (D2) of thecompressed packet Pb(2) with reference˜to the reference data (D1) storedin the management unit 25 and the difference data (D1−D2) stored in thecompressed packet Pb(2). Thereafter, the transmission data (D2) istransmitted as the restored data Irs of the difference data (D1−D2) fromthe restoration unit 23 to the output unit 26, and the transmission data(D2) is output from the output unit 26.

Thereafter, when the compressed packets Pb(3) and Pb(4) are input to thepacket restoration unit 23 as normal packets Pno, the transmission data(D3) and (D4) are generated as restored data Irs corresponding to thedifference data (D1−D3) and (D1−D4), respectively, in the same manner asdescribed for the compressed packet Pb(2). These transmission data (D3)and (D4) are output from the output unit 26.

Further, when either the identifier (ID=1) which is stored in the normalpacket Pno (compressed packet) inputted to the restoration unit 23 orthe corresponding reference data (D1) is not stored in the managementunit 25, the restoration unit 23 discards the normal packet Pno(compressed packet) and outputs an error signal Se indicating that arestoration error has occurred, to the error notification unit 24.

On receipt of the error signal Se, the notification unit 24 notifies thetransmitting end that the restoration error has occurred at thereceiving end, by a restoration error notification signal Ne.

FIG. 7 is a flowchart for explaining the procedure performed in thepacket restoration unit 23.

When a normal packet Pno is transmitted from the error packet detectionunit 22 to the packet restoration unit 23 (step Sb1), it is detectedwhether the normal packet Pno is an uncompressed packet or a compressedpacket (step Sb2).

When the normal packet Pno is the uncompressed packet Pa, theuncompressed packet Pa is subjected restoration, i.e., the transmissiondata (D) is taken from the data section Dpa of the uncompressed packetPa (step Sb6). Then, the identifier (ID) and the reference data (D) asthe receiving-end reference information Im2 are updated according to aninstruction from the packet restoration unit 23 (receiving-endmanagement control signal Cm2) (step Sb7). Further, the transmissiondata (D) taken from the data section dpa of the uncompressed packet pais sent to the output unit 26 (step Sb10).

On the other hand, when the normal packet Pno is the compressed packetpb, the packet restoration unit 23 inquires of the reference informationmanagement unit 25 as to whether the identifier (ID) and the referencedata (D) are stored as the receiving-end reference information Im2 inthe management unit 25 (steps Sb3).

Next, it is decided whether the uncompressed packet (reference packet)required for restoration of the difference data in the compressed packethas been received (step Sb4). This decision is made by collating theidentifier (ID) stored as the reference packet identifier Ih2 b in thecompressed packet Pb and the corresponding transmission data (D), withthe identifier (ID) stored in the reference information management unit25 and the corresponding reference data (D).

When the uncompressed packet (reference packet) Pa for the compressedpacket Pb has been received, the transmission data (D) of the compressedpacket Pb is restored using the reference data (D) stored in thereference information management unit 25 (step Sb5). Further, therestored transmission data (D) is output to the output unit 26 (stepSb10). Thereafter, the restoration unit 23 returns the process of stepSb2.

Based on the result of the decision in step Sb4, when the uncompressedpacket (reference packet) for the received compressed packet has notbeen received, the packet restoration unit 23 discards the compressedpacket Pb which is the received normal packet Pno (step SB8). Then, therestoration unit 23 outputs an error signal Se to the error notificationunit 24. Thereafter, the restoration unit 23 returns to the process ofstep Sb2.

These process steps by the restoration unit 23 are continued until thelast packet is received.

As described above, according to the data transmission method of thefirst embodiment, when performing packet-by-packet data transmission byusing uncompressed packets Pa each containing uncompressed transmissiondata and compressed packets each containing compressed transmissiondata, difference data (ΔD) between the transmission data of a compressedpacket Pb to be transmitted and transmission data of an uncompressedpacket Pa which has been transmitted most-recently is stored in thecompressed packet Pb as compressed data Id. Therefore, so long as theuncompressed packet Pa has been normally transmitted, even when atransmission error occurs in some compressed packet Pb, the differencedata (ΔD) of compressed packets Pb which are normally transmitted afterthe error packet can be restored using the transmission data of theuncompressed packet Pa. Therefore, the number of compressed packets tobe discarded due to the transmission error in the compressed packet issignificantly reduced. As the result, the quality of data transmitted inthe radio section is improved. In other words, the effective rate ofdata transmission is improved, and the time and cost required fortransmission of unrestorable packets are significantly reduced.

While in this first embodiment only one reference packet identifier (ID)Ih2 b is included in the header section Hpb of each compressed packetPb, a plurality of reference packet identifiers (ID) may be included inthe header section Hpb. In this case, however, a plurality ofuncompressed packet must be transmitted continuously.

(Modification 1 of Embodiment 1)

FIG. 8(a) shows the data structure of an uncompressed packet Paa whichis used when a compressed packet includes two reference packetidentifiers (ID). In this case, the uncompressed packet Paa istransmitted twice continuously.

The uncompressed packet Paa is composed of a header section Hpaa whichcontains header information, and a data section Dpaa which containstransmission data (D) to be transmitted as uncompressed data Ir by PPP.The information in the header section Hpaa is composed of acompression/uncompression identifier Ih1, a packet identifier (ID) Ih2a, and other header information Ih3. The identifier Ih1 of thisuncompressed packet Paa indicates “uncompressed”.

FIG. 8(b) shows the data structure of a compressed packet Pbb includingtwo reference packet identifiers (ID).

This compressed-packet Pbb is composed of a header section Hpbb whichcontains header information, and a data section Dpbb which containsfirst and second compressed data Id1 and Id2 to be transmitted by PPP.The information in the header section Hpbb is composed of acompression/uncompression identifier Ih1, first and second referencepacket identifiers (ID1,ID2) Ih2 b 1 and Ih2 b 2 for identifyinguncompressed packets as reference packets, and other header informationIh3. The identifier Ih1 of this compressed packet Pbb indicates“compressed”. The second compressed data Id2 is difference data (Δ2D)between the transmission data of the compressed packet Pbb andtransmission data of an uncompressed packet which has been transmittedmost-recently. Further, the first compressed data Id1 is difference data(Δ1D) between the transmission data of the compressed packet Pbb andtransmission data of an uncompressed packet Paa which has beentransmitted previously to the most-recently-transmitted uncompressedpacket Paa.

In this case, the transmission data (D1)˜(D4) shown in FIG. 4 aretransmitted as follows.

FIG. 9 shows the flow of plural packets from the transmitting end to thereceiving end in the normal transmission state.

In this first modification of the first embodiment, the transmissiondata (D1) and˜(D2) are not compressed and sequentially transmitted byuncompressed packets Paa(1) and Paa(2), and the transmission data (D3)and (D4) are compressed and sequentially transmitted by compressedpackets Pb(3) and Pb(4) which follow the uncompressed packet Paa(2).

At the transmitting end, initially, the uncompressed packet Paa(1) isgenerated to be transmitted to the receiving end. At this time, thetransmission data (D1) is stored as the uncompressed data Ir in the datasection Dpaa of the uncompressed packet Paa(1). Further, the identifierIh1 indicating “uncompressed”, the packet identifier (ID=0) Ih2 a foridentifying this packet, and other header information Ih3 are stored inthe header section Hpaa of the uncompressed packet Paa(1).

Next, the uncompressed packet Paa(2) is generated to be transmitted tothe receiving end. At this time, the transmission data (D2) is stored asthe uncompressed data Ir in the data section Dpaa of the uncompressedpacket Paa(2). Further, the identifier Ih1 indicating “uncompressed”,the packet identifier (ID=0) Ih2 a for identifying this packet, andother header information Ih3 are stored in the header section Hpaa ofthe uncompressed packet Paa(2).

Thereafter, the compressed packet Pbb(3) is generated to be transmittedto the receiving end. At this time, the first and second compressed dataId1 and ld2 corresponding to the transmission data (D3) are stored inthe data section Dpbb of the compressed packet Pbb(3). Further, theidentifier Ih1 indicating “compressed”, the first reference packetidentifier (ID=0) Ih2 b 1, the second reference packet identifier (ID=1)Ih2 b 2, and other header information Ih3 are stored in the headersection Hpbb of the uncompressed packet Pbb(3).

The first compressed data Id1 is difference data (D1−D3) which isobtained by subtracting the transmission data (D3) of the compressedpacket Pbb(3) from the transmission data (D1) of the uncompressed packetPaa(1) by using this packet Paa(1) as a reference packet. Further, thesecond compressed data Id2 is difference data (D2−D3) which is obtainedby subtracting the transmission data (D3) of the compressed packetPbb(3) from the transmission data (D2) of the uncompressed packet Paa(2)by using this packet Paa(2) as a reference packet.

Further, the compressed packet Pbb(4) is generated to be transmitted tothe receiving end. At this time, the first and second compressed dataId1 and Id2 corresponding to the transmission data (D4) are stored inthe data section Dpbb of the compressed packet Pbb(4). Further, theidentifier Ih1 indicating “compressed”, the first reference packetidentifier (ID=0) Ih2 b 1, the second reference packet identifier (ID=1)Ih2 b 2, and other header information Ih3 are stored in the headersection Hpbb of the uncompressed packet Pbb(4).

The first compressed data Id1 is difference data (D1−D4) which isobtained by subtracting the transmission data (D4) of the compressedpacket Pbb(4) from the transmission data (D1) of the uncompressed packetPaa(1) by using this packet Paa(1) as a reference packet. Further, thesecond compressed data Id2 is difference data (D2−D4) which is obtainedby subtracting the transmission data (D4) of the compressed packetPbb(4) from the transmission data (D2) of the uncompressed packet Paa(2)by using this packet Paa(2) as a reference packet.

The first packet identifier (ID=0) Ih2 b 1 stored in the header sectionHpbb of each of the compressed packets Pbb(3) and Pbb(4) indicates thatthe reference packet is the uncompressed packet Paa(1). Likewise, thesecond packet identifier (ID=1) Ih2 b 2 stored in the header sectionHpbb of each of the compressed packets Pbb(3) and Pbb(4) indicates thatthe reference packet is the uncompressed packet Paa(2).

The uncompressed packets Paa(1) and Paa(2) and the following compressedpackets Pbb(3) and Pbb(4), which have been sequentially transmitted fromthe transmitting end, are sequentially received at the receiving end inthe normal data transmission state, and the transmission data (D1)˜(D4)corresponding to the respective packets are restored.

That is, at the receiving end, when the uncompressed packets Paa(1) andpaa(2) are received, the transmission data (D1) and (D2) are taken fromthe data sections Dpaa. When the compressed packet Pbb(3) is received atthe receiving end, the second difference data (D2−D3) is taken from thedata section Dpbb, and the transmission data (D3) of the compressedpacket Pbb(3) is restored from the difference data (D2−D3) withreference to the transmission data (D2) of the second uncompressedpacket Paa(2) which is specified by the second reference packetidentifier (ID=1) Ih2 b 2.

Thereafter, when the compressed packet Pbb(4) is received at thereceiving end, in like manner as described for the compressed packetPbb(3), the difference data (D2−D4) is taken from its data section Dpbb,and the transmission data (D4) of the compressed packet Pbb(4) isrestored from the difference data (D2−D4) with reference to thetransmission data (D2) of the uncompressed packet Paa(2) which isspecified by the second reference packet identifier (ID=1) Ih2 b 2.

Since the uncompressed packets Paa(1) and Paa(2) which are referencepackets for the compressed packets Pbb(3) and Pbb(4) are normallyreceived, the uncompressed packet Paa(2) which is nearer to thesecompressed packets is used as a reference packet. However, when atransmission error occurs in the uncompressed packet Paa(2), theprevious uncompressed packet Paa(1) is used as a reference packet toperform restoration of the compressed packets.

As described above, in this first modification of the first embodiment,two uncompressed packets Paa are continuously transmitted and,thereafter, a compressed packet Pbb is transmitted, which containsdifference data (A1D) and (A2D) based on the transmission data of theseuncompressed packets, and first and second reference packet identifiers(ID1,ID2) Ih2 b 1 and Ih2 b 2 which indicate that the uncompressedpackets Paa are reference packets. Therefore, the compressed packet canbe restored so long as at least one of the identifiers (ID1) and (ID2)in the compressed packet and the reference data corresponding to thisidentifier are stored in the reference information management unit 25.In other words, the number of compressed packets to be discarded due toa transmission error of an uncompressed packet is reduced.

In the first embodiment, the compression/uncompression decision unit 13in the data transmission apparatus 101 controls the packet formationunit 12 so that an uncompressed packet is formed immediately aftercommunication has started or a restoration error signal Ne from theerror notification reception unit 14 has been received and, thereafter,compressed packets are continuously formed until a next restorationerror signal Ne is received. However, the construction of the decisionunit 13 is not restricted thereto, and it may control the packetformation unit 12 so that an uncompressed packet is transmittedperiodically.

In this case, in the state where there is no restoration errornotification from the receiving end, the decision unit 13 instructs thepacket formation unit 12 to from one uncompressed packet every time apredetermined number of compressed packet have been transmitted. Forexample, when the number of compressed packets, which is predeterminedas the uncompressed packet transmission cycle, is three, transmission ofone uncompressed packet and transmission of following three compressedpackets are repeated.

Hereinafter, the effect obtained by the above-described constructionwill be described briefly.

Video data, audio data, and header information of a TCP/IP or UDP/IPpacket required to transmit these data, are stored as transmission datain the data sections Dpa and Dpb of the PPP packets to be transmitted bythe PPP (i.e., the packets shown in FIGS. 1(a) and 1(b)).

Although a difference (difference data) in the video data, the audiodata, or the header information between two adjacent packets is verysmall or 0 in many cases, a difference between distant packets tends tobe large. Therefore, by periodically transmitting an uncompressedpacket, the quality of data transmitted by radio is improved, and theaverage of the above-described difference data is reduced, that is, thecompression efficiency of data in the data section is improved.

Further, the compression/uncompression decision unit 13 in the datatransmission apparatus 101 of the first embodiment may be constructed asfollows. That is, the decision unit 13 obtains the average size m ofdifference data stored in the data section of each compressed packet,and controls the packet formation unit 12 so as to transmit anuncompressed packet when this size m exceeds a predetermined value x.

The average size m is obtained by averaging the difference data ofplural compressed packets which have been transmitted from when thelatest uncompressed packet was transmitted to the present point of time.More specifically, when four compressed packets have been transmittedfrom when the latest uncompressed packet was transmitted to the presentpoint of time and the sizes of difference data of these four compressedpackets are “2”, “4”, “4”, and “6”, respectively, the average size m ofdifference data at the present point of time is 4 (=(2+4+4+6)/4).

Also in this case, the quality of data transmitted by radio is improved,and the average of difference data is reduced, that is, the compressionefficiency of data in the data section is improved.

Measurement of the average size m of the difference data may beperformed in the data receiving apparatus 201.

To be specific, at the receiving end, the packet restoration unit 23measures the average size m of difference data, and outputs a sizeexcess signal to the error notification transmission unit 24 when theaverage size m exceeds a predetermined value x. Further, the errornotification transmission unit 24 outputs a restoration error signal Neto the transmitting end on receipt of the error signal Se and, moreover,it outputs a request signal for transmission of an uncompressed packet,to the transmitting end, on receipt of the size excess signal.

At the transmitting end, the error notification reception unit 14 in thedata transmission apparatus 101 outputs an error notification receptionsignal Sn to the compression/uncompression decision unit 13 not onlywhen receiving the restoration error signal Ne but also when receivingthe packet request signal.

Further, transmission of the uncompressed packet may be performed notwhen the average size m of difference data exceeds the predeterminedvalue x but when a compressed packet in which the size of differencedata exceeds a predetermined value x is transmitted or received.

For example, in the data transmission apparatus 101, when the size ofcompressed data included in a compressed packet to be transmittedexceeds a predetermined value, an uncompressed packet is transmittedsubsequently to this compressed packet.

Further, in the data reception apparatus 201, when the size ofcompressed data included in a compressed packet to be restored exceeds apredetermined value, a request for transmission of an uncompressedpacket is output to the transmitting end. In the data transmissionapparatus 101, on receipt of this request from the receiving end, anuncompressed packet is transmitted to the receiving end.

(Modification 2 of Embodiment 1)

In the first embodiment, as shown in FIGS. 1(a) and 1(b), the datasection Dpb of the compressed packet Pb contains difference data (ΔD)between the whole transmission data of this compressed packet Pb and thewhole transmission data of the uncompressed packet Pa. However, the datasection Dpb of the compressed packet Pb may contain data obtained bycompressing only a part of the transmission data of the compressedpacket.

That is, the transmission data is separated into data to be compressed(hereinafter referred to as “compression target data” or “target data”)and data not to be compressed (hereinafter referred to as “non-targetdata”), and difference data between the compression target data of theuncompressed packet and the compression target data of the compressedpacket is stored in the data section of the compressed packet and,further, the non-target data of the compressed packet is also stored inthe data section.

FIG. 10(a) shows the data structure of an uncompressed packet Pc whichis used when transmission data is composed of compression target dataand non-target data.

The uncompressed packet Pc is composed of a header section Hpccontaining header information, and a data section Dpc containinguncompressed data Ir to be transmitted by PPP. The information in theheader section Hpc is composed of a compression/uncompression identifierIh1 indicating whether the data in the data section is compressed, apacket identifier (ID) Ih2 a for identifying this uncompressed packet,and other header information Ih3. In the data section Dpc, compressiontarget data which is not compressed (hereinafter referred to asuncompressed target data), and non-target data Inc are stored. Theuncompressed target data is composed of three pieces of item-basisuncompressed data Ira, Irb, Ire corresponding to first, second, andthird items to be compressed (hereinafter referred to as target items).More specifically, the three pieces of item-basis uncompressed data Ira,Irb, and Irc are transmission data (Da), (Db), and (Dc) corresponding tothe first, second, and third target items in the uncompressed packet.

FIG. 10(b) shows the data structure of a compressed packet Pd which isused when transmission data is composed of compression target data andnon-target data.

The compressed packet Pd is composed of a header section Hpd containingheader information, and a data section Dpd containing partiallycompressed data to be transmitted by PPP. The information in the headersection Hpd is composed of a compression/uncompression identifier Ih1indicating whether the data in the data section is compressed, areference packet identifier (ID) Ih2 b for identifying a referencepacket, and other header information Ih3.

In the data section Dpd, compression target data which is compressed(hereinafter referred to as compressed target data) and non-target dataInc are stored. The compressed target data is composed of three piecesof item-basis compressed data Ida, Idb, and Idc corresponding to first,second, and third items to be compressed (hereinafter referred to as“target items”). To be specific, the compressed data Ida is differencedata (ΔDa) between the data (Da) corresponding to the first target itemin the transmission data of the uncompressed packet and the data (Da)corresponding to the first target item in the transmission data of thecompressed packet. The compressed data Idb is difference data (ΔDb)between the data (Db) corresponding to the second target item in thetransmission data of the uncompressed packet and the data (Db)corresponding to the second target item in the transmission data of thecompressed packet. The compressed data Idc is difference data (ΔDc)between the data (Dc) corresponding to the third target item in thetransmission data of the uncompressed packet and the data (Dc)corresponding to the third target item in the transmission data of thecompressed packet.

In this case, in the data transmission apparatus 101, the referenceinformation management unit 15 tables the reference packet identifier(ID), the respective compression target items, and the datacorresponding to the respective target items in the reference data(item-basis reference data), and stores this table.

Further, also in the data reception apparatus 201, the referenceinformation management unit 25 tables the reference packet identifier(ID), the respective compression target items, and the item-basisreference data, and stores the table.

In the above-described construction, since the transmission data iscompressed for each compression target item, the storage capacity of amemory, such as a RAM, mounted on the management unit 15 or 25 can bereduced while maintaining the effect of reducing a predeterminedquantity of data by data compression.

(Modification 3 of Embodiment 1)

In the above-described first embodiment and the first and secondmodifications thereof, the data section of each compressed packetcontains, as data obtained by compressing the whole or a part oftransmission data of the compressed packet, difference data between thewhole or a part of the transmission data of the uncompressed packet andthe whole or a part of the transmission data of the compressed packet.However, instead of the difference data or in addition to the differencedata, difference-specifying additional information (K) for calculatingthe difference data may be stored in the header section or the datasection of the compressed packet.

For example, in the compressed packet Pb shown in figure 1(b), in steadof the difference data, difference-specifying additional information (K)for calculating this difference data may be stored.

In the compressed packet Pd shown in FIG. 10(b), instead of thedifference data corresponding to at least one item-basis compressed dataamongst the plural pieces of item-basis compressed data,difference-specifying additional information (K) for calculating thisdifference data may be stored.

FIGS. 11(a) and 11(b) are diagrams for explaining the data structure ofa packet which is used in the case where difference-specifyingadditional information (K) is stored in a compressed packet.

FIGS. 11(a) and 11(b) illustrate an uncompressed packet Pe and acompressed packet Pf to be used in this case, respectively.

The uncompressed packet Pe is composed of a header section Hpecontaining header information, and a data section Dpe containingtransmission data to be transmitted by PPP. The packet Pe has the samestructure as the uncompressed packet Pc shown in FIG. 10(a).

The compressed packet Pf is composed of a header section Hpf containingheader information, and a data section Dpf containing partiallycompressed data to be transmitted by PPP. The header section Hpfcontains difference-specifying additional information (K) Ih4 inaddition to a compression/uncompression identifier Ih1, a referencepacket identifier (ID) Ih2 b, and other header information Ih3 which areidentical to those mentioned for the compressed packet Pd shown in FIG.10(b). Further, the data section Dpf contains three pieces of item-basiscompressed data Ida, Idb, and Idc corresponding to first, second, andthird items, and non-target data Inc, which are identical to thosementioned for the compressed packet Pd shown in FIG. 10(b).

The difference-specifying additional information (K) is a sequencenumber indicating the position of the compressed packet, counted fromthe uncompressed packet which is referred to for restoration of thecompressed packet. Further, difference data (ΔDa) and (ΔDb) as theitem-basis compressed data Ida and Idb are equal to thedifference-specifying additional information (K) and, therefore, thedata size of the difference data (ΔDa) and (ΔDb) is 0 byte.

Hereinafter, a description will be given of the case where thedifference-specifying additional information (K) is stored in thecompressed packet, taking data transmission using RTP (Real TimeProtocol) as an example.

To be specific, a description will be given of the case where video dataor audio data is converted to RTP type data according to RTP defined inRFC1889/1890, and the RTP type data is converted to UDP/IP type dataaccording to UDP and IP, and then the RTP/UDP/IP type data istransmitted from the data transmission terminal 101 to the datareception terminal 201. The RTP/UDP/IP type data corresponds to the IPpacket Pipb shown in FIG. 29(d).

Usually, the sequence number Isn included in the header Hrtp of the RTPpacket Prtp (refer to FIG. 29(a)) increments by 1 every time one RTPpacket is formed. Further, the packet ID (IPv4 (Internet Protocolversion 4) ID, not shown) included in the header Hipb of the IP packetPipb also increments by 1 every time one IP packet is formed. When thesevalues are stored as difference data in a compressed packet, thesevalues can be set at 0 if the position of the compressed packet, countedfrom an uncompressed packet as a reference packet for the compressedpacket, can be detected.

In other words, when the sequence number Isn in the header section Hrtpof the RTP packet Prtp is stored as simple difference data in thecompressed packet, at least 1 byte is always needed as the data quantityof the sequence number Isn. However, by using the above-describeddifference-specifying additional information, the size of the differencedata corresponding to the sequence number Isn becomes 0 byte, wherebythe compression efficiency is improved.

For example, when the size of the difference-specifying additionalinformation (K) is 1 byte, the size of the difference data of thesequence number Isn in the header section Hrtp of the RTP packet Prtp isusually 0 byte, and the sum of the difference-specifying additionalinformation and the difference data is usually 1 byte. In this case,even when the difference-specifying additional information (K) is used,the data quantity of the RTP packet does not change.

However, when plural pieces-of compression target data which can berestored using the same calculation method as the above-described one(i.e., addition of the difference-specifying additional information andthe difference data) are included in the transmission data, for example,when there are two kinds of information such as the sequence number inthe header section of the RTP packet and the IPv4 ID in the headersection of the IP packet, substantial effect is obtained by using thedifference-specifying additional information, whereby the compressionefficiency is significantly improved.

Further, an arithmetic expression having the above-describeddifference-specifying additional information as a variable may be usedfor obtaining difference data of a compressed packet to be processed,from transmission data of a reference packet (uncompressed packet) usedfor restoration of the compressed part.

As for the arithmetic expression, for example, there is an expressionwhich defines four rules (addition, subtraction, multiplication,division) or functional arithmetic such as sin and cos.

Further, the arithmetic expression having the difference-specifyingadditional information as a variable may be dynamically changed duringdata transmission according to a predetermined rule even though it ispreviously decided at the transmitting end and the receiving end.Thereby, the compression efficiency of transmission data stored in thedata section of the PPP packet is further improved, and the quality ofdata transmitted by radio and the effective transmission rate arefurther improved.

[Embodiment 2]

FIG. 12 is a block diagram for explaining a data transmission methodaccording to a second embodiment of the present invention, illustratinga data transmission apparatus 102 in a data transmission system usingthis data transmission method. This second embodiment corresponds toaspects 1, 2, 10˜12, 18˜21, 32, 33, 36, and 37.

The data transmission apparatus 102 includes, in addition to theconstituents of the data transmission apparatus 101 of the firstembodiment, a monitor unit 31 which monitors the number of times that anuncompressed packet output from the compressed/uncompressed packetformation unit 12 is transmitted to the receiving end. The monitor unit31 receives packets from the packet formation unit 12, and continuouslyoutputs the same uncompressed packet Pa by a predetermined number oftimes (in the second embodiment, two times) to the packet transmissionunit 16, and then outputs compressed packets Pb which follow theuncompressed packet Pa, to the packet transmission unit 16. Otherconstituents of the data transmission apparatus 102 are identical tothose of the data transmission apparatus 101 of the first embodiment.

A data reception apparatus in the data transmission system of thissecond embodiment is identical to the data reception apparatus 201 inthe data transmission system of the first embodiment.

Next, the function and effect will be described.

In the data transmission apparatus 102 of this second embodiment, thenumber of times that the uncompressed packet Pa is transmitted ismonitored by the monitor unit 31. For example, as shown in FIG. 13, whenan uncompressed packet Pa(1) is output from the packet formation unit 12to the monitor unit 31, the monitor unit 31 outputs the sameuncompressed packet Pa(1) twice to the packet transmission unit 16.Thereafter, compressed packets Pb(2), Pb(3), and Pb(4) which follow theuncompressed packet Pb(1) are sequentially output to the packettransmission unit 16. In the packet transmission unit 16, those packetssupplied from the monitor unit 31 are sequentially output by apredetermined radio transmission method such as W-CDMA.

The other constituents of the data transmission apparatus 102 operate inthe same manner as described for the first embodiment.

On the other hand, in the data reception apparatus, when thecontinuously-transmitted two uncompressed packets Pa(1) are normallyreceived, the reference information management unit 25 updates theidentifier (ID) and the reference data (D). Therefore, when thefollowing compressed packets Pb(2), Pb(3), and Pb.(4) are received, theidentifier (ID=0) and the reference data (D1) stored in the managementunit 25 are referred to.

Even when one of the two uncompressed packets Pa(1) has not arrived atthe receiving end due to a transmission error, the normally transmittedpacket Pa(1) is input to the packet restoration unit 23 through theerror packet detection unit 22. Therefore, in the reference informationmanagement unit 25, the identifier (ID) and the reference data (D) whichare the receiving-end reference information Im2 are updated to thosecorresponding to the uncompressed packet Pa(1).

As described above, according to the second embodiment of the invention,the uncompressed packet Pa is continuously transmitted twice and,thereafter, the following compressed packets Pb are transmitted. So,even when a transmission error occurs in one of the two uncompressedpackets Pa, the difference data of the following compressed packets arenormally restored at the receiving end. Therefore, the number ofreceived packets to be discarded due to a restoration error at thereceiving end is reduced, whereby the quality of data transmitted byradio is improved.

While in this second embodiment the uncompressed packet is transmittedtwice, it may be transmitted three time or more.

Further, in this second embodiment, the monitor unit 31 managestransmission of the uncompressed packet so that the uncompressed packetitself is transmitted by plural times. However, the monitor unit 31 maycontrol, after transmission of the uncompressed packet, transmission ofan auxiliary packet so that it is transmitted by a predetermined numberof times (at least one time), which auxiliary packet is different fromthe uncompressed packet and contains the packet identifier (ID) and thetransmission data (D) of the uncompressed packet.

In this case, the packet formation unit 12 forms, after formation of theuncompressed packet, an auxiliary packet which contains the packetidentifier (ID) and transmission data (D) of the uncompressed packet.Thereafter, a plurality of compressed packets based on the uncompressedpacket (i.e., compressed packets containing different data obtained byusing the transmission data of the uncompressed packet) are formed. Theuncompressed packet, the auxiliary packet, and the compressed packetsare supplied to the monitor unit 31 in this order. In the monitor unit31, initially the uncompressed packet is transmitted and then theauxiliary packet is transmitted by a predetermined number of times.Thereafter, the compressed packets are sequentially transmitted.

In this construction, by using one uncompressed packet and apredetermined number (at least one) of auxiliary packets, the referencepacket identifier and the reference data are transmitted at least twotimes before the compressed packets based on the uncompressed packet aretransmitted. Therefore, even when a transmission error occurs in any ofthese uncompressed packet and auxiliary packets, the difference data ofthe subsequent compressed packets are normally restored at the receivingend.

Thereby, the number of received packets to be discarded due to arestoration error at the receiving end is reduced, and the quality ofdata transmitted by radio is improved.

While in this second embodiment the uncompressed packet or the auxiliarypacket is transmitted by a predetermined number of times, the number oftimes may be changed according to the frequency of restoration errornotification which is sent from the receiving end to the transmittingend.

For example, when the number of times that the uncompressed packet istransmitted is changed, in the compression/uncompression decision unit13, the number of times per unit time that the error notificationreception signal Sn from the error notification reception unit 14 isinput, is counted, and this count is compared with a predeterminedreference value Y. According to the result of the comparison, a controlsignal for controlling the number of transmission times is output to themonitor unit 31. In the monitor unit 31, on the basis of this controlsignal, the number of times that the uncompressed packet or theauxiliary packet is transmitted is increased or decreased. To bespecific, when the count exceeds the reference value Y, the number oftransmission times is increased, and when the count becomes equal to orlower than the reference value Y, it is decreased.

In this construction, when the quality of the transmission data isrelatively stable, the transmission efficiency can be improved bydecreasing the number of times that the uncompressed packet or theauxiliary packet is transmitted. When the quality of the transmissiondata is unstable, the number of packets to be discarded at the receivingend due to a restoration error can be reduced by increasing the numberof transmission times.

[Embodiment 3]

FIGS. 14 and 15 are diagrams for explaining a data transmission methodaccording to a third embodiment of the present invention. This thirdembodiment corresponds to aspects 1, 2, 13˜15, 18˜21, 32, 33, 36, and37.

FIG. 14 is a block diagram illustrating a data transmission apparatus103 in a data transmission system which performs data transmission bythe data transmission method.

The data transmission apparatus 103 includes, in addition to theconstituents of the data transmission apparatus 101 of the firstembodiment, an ECC (Error Correction Code) addition unit 32 whichreceives an uncompressed packet Pa and a compressed packet Pb outputfrom the compressed/uncompressed packet formation unit 12, and gives anECC to the uncompressed packet Pa. The ECC-added uncompressed packet Pacwhich is obtained in the unit 32 and the compressed packet Pb which haspassed the unit 32 are input to the packet transmission unit 16. Otherconstituents of the data transmission apparatus 103 are identical tothose of the data transmission apparatus 101 of the first embodiment.

FIG. 15 is a block diagram illustrating a data reception apparatus 203in the data transmission system which performs data transmission by thedata transmission method of this third embodiment.

The data reception apparatus 203 of this third embodiment includes, inaddition to the constituents of the data reception apparatus 201 of thefirst embodiment, an error correction unit 41 which receives the packetsRp output from the packet reception unit 21, and performs errorcorrection on the ECC-added uncompressed packet Pac. The errorcorrection unit 41 outputs the compressed packet to which no ECC isadded, as it is. The packets output from the error correction unit 41are input to the error packet detection unit 22. Other constituents ofthe data reception apparatus 203 are identical to those of the datareception apparatus 201 of the first embodiment.

Next, the function and effect will be described.

In the data transmission apparatus 103 constructed as described above,when the uncompressed packet Pa formed in the packet formation unit 12(i.e., the packet containing reference data to be used for restorationof the compressed packets) is input to the ECC addition unit 32, the ECCaddition unit 32 adds an ECC to the uncompressed packet Pa, and outputsthe ECC-added uncompressed packet Pac to the packet transmission unit16. When the compressed packet Pb formed in the packet formation unit 12is input to the ECC addition unit 32, the unit 32 does not process thiscompressed packet Pb, and outputs it to the packet transmission unit 16.Other constituents of the data transmission apparatus 103 operate in thesame manner as described for the data transmission apparatus 101 of thefirst embodiment.

On the other hand, in the data reception unit 203, when the receivedpackets Rp output from the packet reception unit 21 are input to theerror correction unit 41, the ECC-added uncompressed packet Pac issubjected to error correction and output to the error packet detectionunit 22, while the compressed packet Pb to which no ECC is added isoutput as it is to the error packet detection unit 22. Otherconstituents of the data reception unit 203 operate in the same manneras described for the data reception unit 201 of the first embodiment.

As described above, according to the third embodiment, an ECC is addedto the uncompressed packet Pa at the transmitting end, and the ECC-addeduncompressed packet Pac is transmitted to the receiving end. At thereceiving end, the ECC-added uncompressed packet Pac is subjected toerror correction using the ECC. Therefore, even when transmission errorsoccur, most uncompressed packets are recovered at the receiving end,thereby suppressing occurrence of defective uncompressed packets due totransmission errors.

Therefore, the number of received packets to be discarded due to arestoration error in the compressed packet that follows the uncompressedpacket is reduced, and the quality of data transmitted by radio isimproved.

While in this third embodiment an ECC is added to the uncompressedpacket itself, an ECC may be added to a part of the uncompressed packet,i.e., a part including reference information (identifier (ID) andreference data (D)) which is required for restoration of the followingcompressed packets.

In this case, at least the identifier (ID) and the reference data (D)which are required for restoration of the compressed packets, aresubjected to error correction at the receiving end.

Thereby, the number of received packets to be discarded due to arestoration error in the compressed packet which follows theuncompressed packet is reduced, and the quality of data transmitted byradio is improved.

Further, while in this third embodiment an ECC is added to everyuncompressed packet, it may be decided whether an ECC is to be added tothe uncompressed packet, according to the frequency of restoration errornotification which is performed from the receiving end to thetransmitting end.

In this case, in the compression/uncompression decision unit 13, thenumber of times per unit time that the error notification receptionsignal Sn from the error notification reception unit 14 is input, iscounted, and the count is compared with a predetermined reference valueY. According to the result of the comparison, an error correctioncontrol signal is output to the formation unit 12. According to theerror correction control signal, the formation unit 12 notifies the ECCaddition unit 32 as to whether an ECC is to be added to the uncompressedpacket. To be specific, when the count exceeds the reference value Y,the ECC addition unit 32 adds an ECC to the uncompressed packet andoutputs it. When the count is equal to or lower than the reference valueY, the ECC addition unit 32 adds no ECC to the uncompressed packet, andoutputs the packet as it is.

In this construction, when the quality of transmission data isrelatively stable, the effective transmission rate is increased bytransmitting the uncompressed packet as it is. On the other hand, whenthe quality of transmission data is unstable, the number of packets tobe discarded at the receiving end due to a restoration error is reducedby adding an ECC to the uncompressed packet.

[Embodiment 4]

FIGS. 16 and 17 are block diagrams for explaining a data transmissionmethod according to a fourth embodiment of the present invention. FIG.16 illustrates a data transmission apparatus 104 in a data transmissionsystem which employs the data transmission method. This fourthembodiment corresponds to aspects 1, 2, 16˜21, 32, 33, 36 and 37.

The data transmission apparatus 104 includes, instead of the errornotification reception unit 14 according to the first embodiment, aretransmission request notification reception unit 14 d which receives arequest signal for retransmission of an uncompressed packet(retransmission request signal Nr) from the receiving end, and outputs aretransmission request reception signal Sr. Further, the constructionsof the packet formation unit 12 and the compression/uncompressiondecision unit 13 are altered so that uncompressed packets are formedaccording to the retransmission request reception signal Sr.

To be specific, in this data transmission apparatus 104, thecompression/uncompression decision unit 13 d outputs a packet decisionsignal Jp indicating the type of a packet to be formed next by thepacket formation unit 12 d, to the packet formation unit 12 d. When thedecision unit 13 d receives the retransmission request reception signalSr, it outputs, instead of the packet decision signal Jp, a re-forminstruction signal sc which instructs the formation unit 12 d to re-formthe uncompressed packet for which retransmission is requested. Further,the packet formation unit 12 d forms either an uncompressed packet or acompressed packet on the basis of the packet decision signal Jp. Onreceipt of the re-form instruction signal Sc, the formation unit 12 dre-forms the uncompressed packet which has been formed most-recently, onthe basis of the identifier (ID) and the reference data (D) stored inthe reference information management unit 15. The retransmission requestsignal Nr, the retransmission request reception signal Sr, and there-form instruction signal Sc include the identifier (ID) whichspecifies the uncompressed packet to be retransmitted.

Other constituents of the data transmission apparatus 104 are identicalto those of the data transmission apparatus 101 of the first embodiment.

FIG. 17 shows a data reception apparatus 204 in the data transmissionsystem of this fourth embodiment.

The data reception apparatus 204 includes, in addition to theconstituents of the data reception apparatus 201 of the firstembodiment, a restoration wait data storage unit 42 which temporarilystores a compressed packet that is decided as a restoration error packetPre, amongst the received compressed packets. Further, the errornotification transmission unit 24 and the packet restoration unit 23according to the first embodiment are altered so that the compressedpacket which is decided as a restoration error packet Pre is subjectedto restoration on the basis of the reference data of the retransmitteduncompressed packet.

That is, the data reception apparatus 204 includes, instead of the errornotification transmission unit 24 of the first embodiment, aretransmission request transmission unit 24 d which outputs, to thetransmitting end, a signal for requesting retransmission of anuncompressed packet which is a reference packet required for restorationof the restoration error packet (retransmission request signal), on thebasis of an error signal Sre which is output when a restoration erroroccurs.

Further, when the receiving-end reference information Im2 (identifier(ID) and reference data (D)) which is required for restoration of thereceived packet is not stored in the reference information managementunit 25, the packet restoration unit 23 d decides that the receivedcompressed packet is a restoration error packet, and outputs an errorsignal Sre to the retransmission request transmission unit 24 d. Theerror signal Sre includes the identifier (ID) which specifies theuncompressed packet to be retransmitted.

In this data reception apparatus 204, the compressed packet which isdecided as a restoration error packet Pre is subjected to restoration onthe basis of the identifier (ID) and the transmission data (D) of theretransmitted uncompressed packet.

Other constituents of the data reception apparatus 204 are identical tothose of the data reception apparatus 201 according to the firstembodiment.

Next, the function and effect will be described.

In the data transmission system according to the fourth embodiment, whena restoration error occurs in the compressed packet Pb, the uncompressedpacket is retransmitted from the transmitting end, in response to theretransmission request signal Nr from the receiving end.

That is, at the receiving end, in the packet restoration unit 23 d, whenit is decided that the compressed packet is a restoration error packetbecause the identifier (ID) and the reference data (D), which are thereceiving-end reference information Im2 required for restoration of thecompressed packet Pb, are not stored in the reference informationmanagement unit 25, this compressed packet Pre is output from therestoration unit 23 d and input to the restoration wait data storageunit 42, wherein the compressed packet Pre is temporarily stored. Atthis time, an error signal Sre including the reference packet identifier(ID) of the restoration error packet Pre is output to the retransmissionrequest transmission unit 24 d. Then, the retransmission requesttransmission unit 24 d transmits a retransmission request signal Nrincluding the reference packet identifier (ID) to the transmitting end.

In the data transmission apparatus 104, when the retransmission requestsignal Nr including the reference packet identifier (ID) is received bythe reception unit 14 d, the reception unit 14 d outputs aretransmission request reception signal Sr to the decision unit 13 d,and the decision unit 13 outputs a signal Sc instructing formation ofthe uncompressed packet specified by the reference packet identifier(re-form instruction signal) to the packet formation unit 12 d. In thepacket formation unit 12 d, the uncompressed packet required forrestoration of the restoration error packet is re-formed on the basis ofthe identifier (ID) and the reference data (D) as the transmission-endreference information Im1 stored in the reference information managementunit 15, and the uncompressed packet so formed is transmitted to thereceiving end through the packet transmission unit 16.

At the receiving end, when the retransmitted uncompressed packet isreceived by the packet reception unit 21, it is supplied to the packetrestoration unit 23 d through the error packet detection unit 22. In thepacket restoration unit 23 d, the identifier (ID) and the transmissiondata (D) are taken from the retransmitted uncompressed packet, and thedifference data (ΔD) of the compressed packet stored in the restorationwait data storage unit 42 is restored on the basis of the identifier(ID) and the transmission data (D).

On the other hand, the identifier (ID) and the transmission data (D) aresupplied to the reference information management unit 25, whereby theidentifier (ID) and the reference data (D) as the receiving-endreference information Im2 are updated.

As described above, according to the fourth embodiment, when arestoration error occurs in a compressed packet, the transmitting endretransmits an uncompressed packet which is required for restoration ofthe compressed packet, according to a retransmission request signal Nrfrom the receiving end. Therefore, even when the received compressedpacket is decided as a restoration error packet because thereceiving-end reference information (identifier (ID) and reference data(D)) required for restoration of the compressed packet is absent at thereceiving end, the restoration error packet can be normally restoredafter completing retransmission of the uncompressed packet. Thereby, thenumber of received packets to be discarded due to the restoration errorof the compressed packet is reduced, and the quality of data transmittedby radio is improved.

While in this fourth embodiment the identifier (ID) and the transmissiondata (D) included in the uncompressed packet are stored in the referenceinformation management unit 15, the uncompressed packet itself may bestored in the management unit 15.

In this case, when performing retransmission of the uncompressed packet,the process of forming the uncompressed packet by the packet formationunit 12 d can be dispensed with.

Further, in this fourth embodiment, the uncompressed packet itself isretransmitted according to the retransmission request from the receivingend. However, on receipt of the request, only a part of the uncompressedpacket including the identifier (ID) and the transmission data (D) maybe retransmitted after storing them in a predetermined packet forretransmission.

Also in this case, the received compressed packet which is decided as arestoration error packet can be restored after transmission of thepacket for retransmission, whereby the number of received packets to bediscarded due to the restoration error of the compressed packet isreduced, and the quality of data transmitted by radio is improved.

Further, while in the first to fourth embodiments difference data (firstdifference data) between transmission data of an uncompressed packet andtransmission data of a compressed packet is stored as compressed data inthe compressed packet, the compressed data to be stored in thecompressed packet may be switched between the first difference data andanother difference data (second difference data) according to thetransmission status of packets.

As an example of the second difference data, there is difference datadefined in the literature by V. Jacobson which is described in thesection of BACKGROUND OF THE INVENTION, that is, difference data betweenthe transmission data of the compressed packet and the transmission dataof the packet which has been formed immediately before the compressedpacket (refer to FIG. 31).

Hereinafter, a description will be given of the case where the datatransmission method in which the compressed data to be stored in thecompressed packet is switched between the first difference data and thesecond difference data according to the transmission status of packets,is applied to the data transmission system according to the firstembodiment, with reference to FIGS. 2 and 3.

In this case, the error notification reception unit 14 in the datareception unit 101 is constructed so as to calculate the frequency (z)of receiving the restoration error signal Ne in a unit time. Further,the compressed/uncompressed packet formation unit 12 is constructed soas to receive a signal indicating the reception frequency (z) calculatedby the error notification reception unit 14. When the receptionfrequency (z) exceeds a predetermined reference value Y, the firstdifference data is formed as the compressed data to be stored in thecompressed packet. On the other hand, when the reception frequency (z)is smaller than the reference value Y, the second difference data isformed as the compressed data.

The operation in this case will be described briefly.

Initially, when an error occurs during the process of restoring thecompressed data included in the compressed packet at the receiving end,the receiving end notifies the transmitting end of this error. At thetransmitting end, when the frequency of error notification from thereceiving end exceeds a predetermined value, the transmitting endrequests the receiving end to change the restoration process to thatusing the first difference data and, thereafter, the transmitting endperforms the compression process using the first difference data. On theother hand, when the frequency of error notification becomes equal to orsmaller than the predetermined value, the transmitting end requests thereceiving end to change the restoration process to that using the seconddifference data and, thereafter, the transmitting end performs thecompression process using the second difference data. Then, thereceiving end performs restoration according to the compression processat the transmitting end.

In this case, an identifier indicating that the compressed data is thefirst difference data or the second difference data may be included inthe compressed packet Pb.

The method of switching the compressed data Id to be stored in thecompressed packet Pb between the first difference data and the seconddifference data, provides the following effects.

Usually, a difference in data, such as the above-mentioned transmissiondata (video or audio data) or the header data, between two adjacentpackets is very small or 0 in many cases, but a difference in such databetween distant packets tends to be large. Therefore, by switching thecompressed data between the first difference data and the seconddifference data, the quality of data transmitted by radio is improvedand, further, the average of difference data is reduced, that is, thecompression efficiency of data stored in the data section is improved.

While in the above-described method the decision about that either thefirst difference data or the second difference data is to be used ismade at the transmitting end, this decision may be made according to aninstruction from the receiving end.

In this case, switching between the first difference data and the seconddifference data may be performed based on the number of transmissionerrors per unit time.

In this case, the number of transmission errors per unit time (incidenceof transmission error) is obtained by the error packet detection unit 22in the data reception apparatus, and the error notification unit 24notifies the transmitting end of the incidence of transmission error.

Further, switching between the first difference data and the seconddifference data may be performed according to the frequency ofrestoration errors at the receiving end.

In this case, the frequency (z) of restoration errors per unit time isobtained in the packet restoration unit 23 at the receiving end, andthis frequency (z) is compared with a predetermined reference value Y.Then, the error notification unit 24 notifies the transmitting end ofthe result of the comparison. At the transmitting end, according to theresult of the comparison, either the first difference data or the seconddifference data is used as the compressed data.

The operation in this case will be described briefly.

At the receiving end, when the frequency of errors in the process ofrestoring the compressed data included in the compressed packet exceedsa predetermined value, the receiving end requests the transmitting endto change the compression process at the transmitting end to that usingthe first difference data. When the frequency of errors becomes equal toor smaller than the predetermined value, the receiving end requests thetransmitting end to change the compression process to that using thesecond difference data.

Then, the transmitting end performs compression using the differencedata according to the request from the receiving end, and the receivingend performs restoration according to the compression process using thedifference data requested to the transmitting end.

[Embodiment 5]

FIGS. 18 to 27 are diagrams for explaining a data transmission methodaccording to a fifth embodiment of the invention, and a datatransmission system using the data as transmission method. This fifthembodiment corresponds to aspects 22˜31, 34, 35, 38˜40.

The data transmission system of this fifth embodiment is a system fortransmitting data in packet units from the transmitting end to thereceiving end. At the transmitting end, when forming a uncompressedpacket containing data to be transmitted (transmission data) and acompressed packet containing compressed transmission data, thetransmission data is compressed by using transmission data (referencedata) corresponding to the uncompressed packet and a specific compressedpacket. At the receiving end, the compressed transmission data isrestored using the reference data.

FIGS. 18(a) and 18(b) are diagrams illustrating data structures(formats) of an uncompressed packet Pg and a compressed packet Ph usedin the data transmission system, respectively.

With reference to FIG. 18(a), the uncompressed packet Pg is composed ofa header section Hpg containing header information, and a data sectionDpg containing uncompressed data Ir to be transmitted by PPP (Point toPoint Protocol). The information stored in the header section Hpg iscomposed of a compression/uncompression identifier Ih1 indicatingwhether the data Ir stored in the data section Dpg is compressed, apacket identifier (ID) for identifying this uncompressed packet, andother header information Ih3. The uncompressed data Ir is transmissiondata (D) to be transmitted by the uncompressed packet.

With reference to FIG. 18(b), the compressed packet Ph is composed of aheader section Hph containing header information, and a data section Dphcontaining compressed data Id to be transmitted by PPP. The informationstored in the header section Hph is composed of acompression/uncompression identifier Ih1 indicating whether the data Idin the data section Dph is compressed, a reference packet identifier(ID) Ih2 b indicating a reference packet which is needed for restorationof the compressed data Id, a reference data updation flag Ih5 indicatingwhether reference data used for the restoration is to be updated, andother header information Ih3.

In an ordinary compressed packet Ph, the reference data updation flagIh5 is set at “Off” indicating that the reference data is not to beupdated. In a specific compressed packet Ph, the updation flag Ih5 isset at “On” indicating that the reference data it to be updated. Thecompressed data Id is difference data (ΔD) between transmission data (D)of a most-recent uncompressed packet or a most-recent specificcompressed packet which has been transmitted previously to thecompressed packet Pb to be transmitted, and transmission data (D) of thecompressed packet Pb to be transmitted.

The header information Ih3 includes a CRC code Icrc shown in FIG. 27(e).

FIG. 19 is a block diagram illustrating a data transmission apparatus105 in the data transmission system according to the fifth embodiment.

The data transmission apparatus 105 includes a reception unit 11, acompressed/uncompressed packet formation unit 12 e, and a packettransmission unit 16, like the data transmission apparatus 101 of thefirst embodiment. The reception unit 11 receives a first transmissionsignal S1 including transmission data (D), and outputs a receptionsignal Src. The packet formation unit 12 e receives the reception signalSrc, and packetizes the transmission data (D) according to a controlsignal, thereby forming an uncompressed packet Pg or a compressed packetPh. The packet transmission unit 16 transmits the packet formed by theformation unit 12 e, as a second transmission signal S2, to thereceiving end.

Further, the data transmission apparatus 105 includes an errornotification reception unit 14 and a compression/uncompression decisionunit 13, like the data transmission apparatus 101. The errornotification reception unit 14 receives a restoration error signal Nefrom the receiving end, and outputs an error notification receptionsignal Sn. The decision unit 13 stores the type of each packet formed bythe packet formation unit 12 e, decides the type of a packet to beformed next on the basis of the stored packet type and the errornotification reception signal Sn, and outputs a packet decision signalJp as a control signal to the packet formation unit 12 e. In the packetformation unit 12 e, either an uncompressed packet Pg or a compressedpacket Ph is formed according to the packet decision signal Jp.

Further, the data transmission apparatus 105 includes a referenceinformation updation decision unit 17. This decision unit 17 stores thetransmission history of compressed packets which have been transmittedto the receiving end, and decides as to whether the reference data is tobe updated, when forming a compressed packet, on the basis of thecompression/uncompression identifier Ih1 and the reference data updationflag Ih5 which are supplied from the packet formation unit 12 e. Everytime the packet formation unit 12 e forms n packets (e.g, threepackets), the decision unit 17 outputs, as the above-described controlsignal, a reference data updation signal-Jr instructing updation of thereference data, to the packet formation unit 12 e. When the updationsignal Jr is input to the packet formation unit 12 e, “On” indicatingthat the reference data is to be updated is stored as the reference dataupdation flag Ih5 in the header section Hph of the compressed packet Ph,whereby a specific compressed packet is formed. On the other hand, whenno updation signal Jr is input to the packet formation unit 12 e, “Off”indicating that the reference data is not to be updated is stored as theupdation flag Ih5 in the header section Hph of the compressed packet Ph,whereby an ordinary compressed packet is formed.

Further, the data transmission apparatus 105 includes a referenceinformation management unit 15 e. This management unit 15 e associatestransmission data (D) to be referred to when forming compressed datacorresponding to each compressed packet with a reference packetidentifier (ID) indicating a reference packet corresponding to thetransmission data (D), and manages them as transmitting-end referenceinformation Im1 (reference data (D) and identifier (ID)). In thismanagement unit 15 e, when an uncompressed packet or a specificcompressed packet including the reference data updation flag “On” Ih5 isformed, the reference data (D) and the identifier (ID) as thetransmitting-end reference information Im1 are updated according to atransmitting-end management control signal Cm1 supplied from the packetformation unit 12 e.

FIG. 20 is a block diagram for explaining a data reception apparatus 205in the data transmission system of this fifth embodiment.

The data reception apparatus 205 includes a packet reception unit 21, anerror packet detection unit 22, a packet restoration unit 23 e, and anoutput unit 26, like the data reception apparatus 201 of the firstembodiment. The packet reception unit 21 receives the packet which hasbeen transmitted from the transmitting end as the second transmissionsignal 52, and outputs the received packet Rp. The error packetdetection unit 22 receives the packet Rp, detects an error packet, andoutputs a normal packet Pno which has been normally transmitted. Thepacket restoration unit 23 e receives the normal packet Pno from thedetection unit 22, and restores the uncompressed data or compressed datastored in the packet. The output unit 26 outputs the restored data Irs(transmission data (D)) as an output signal S3.

The data reception unit 205 includes a reference information managementunit 25 e. This management unit 25 e associates transmission data (D) tobe referred to when restoring compressed data corresponding to eachcompressed packet with a reference packet identifier (ID) indicating areference packet corresponding to the transmission data (D), and managesthem as receiving-end reference information Im2 (reference data (D) andidentifier (ID)). In this management unit 25 e, when the uncompressedpacket or the specific packet including the reference data updation flag“On” Ih5 is restored, the reference data (D) and the identifier (ID) asthe receiving-end reference information Im2 are updated according to areceiving-end management control signal Cm2 supplied from the packetformation unit 23 e.

Further, in the packet restoration unit 23 e, when performingrestoration on the compressed packet Ph, it is decided whether thereference packet identifier (ID) and the corresponding reference data(D) which are stored in the compressed packet Ph are stored in thereference information management unit 25 e. According to the result ofthis decision, an error signal Se which indicates that a restorationerror occurs in the compressed packet, is output.

Further, the data reception unit 205 includes an error notificationtransmission unit 24 which receives the error signal Se from the packetrestoration unit 23 e, and notifies the transmitting end that therestoration error has occurred at the transmitting end, by using arestoration error notification signal Ne.

Next, the function and effect will be described.

FIGS. 21 and 22 are diagrams for explaining the data transmission methodaccording to the fifth embodiment. FIG. 21 shows the flow of pluralpackets from the transmitting end to the receiving end in the normaltransmission state, and FIG. 22 shows the flow of plural packets fromthe transmitting end to the receiving end in the state where atransmission error occurs.

Transmission data (D1)˜(D11) are data which are packetized forpacket-by-packet transmission. In this fifth embodiment, thetransmission data (D1) is not compressed and is transmitted by anuncompressed packet Pg(1). The transmission data (D2)˜(D11) arecompressed and sequentially transmitted by compressed packetsPh(2)˜Ph(11) which follow the uncompressed packet Pg(1), respectively.

At the transmitting end, initially, the uncompressed packet Pg(1) isformed and transmitted to the receiving end. At this time, thetransmission data (D1) is stored as uncompressed data Ir in the datasection Dpg of the uncompressed packet Pg(1). Further, an identifier Ih1indicating “uncompressed”, a packet identifier (ID=0) Ih2 a foridentifying this packet, and other header information Ih3 are stored inthe header section Hpg of this packet Pg(1).

Thereafter, the compressed packets Ph(2)˜Ph(11) are successively formedand transmitted to the receiving end.

When forming these compressed packets, an identifier Ih1 indicating“compressed”, a reference packet identifier (ID=0) Ih2 b, a referencedata updation flag Ih5, and other header information Ih3 are stored inthe header section Hph of each of the compressed packets Ph(2)˜Ph(5).Further, difference data (D1−D2), difference data (D1−D3), differencedata (D1−D4), and difference data (D1−D5) are stored in the datasections Dph of the compressed packets Ph(2)˜Ph(5), respectively.

In this fifth embodiment, the reference data is updated every time threepackets are transmitted. Therefore, in the compressed packetsPh(2)˜Ph(4), the value of the reference data updation flag Ih5 in theheader section Hph is “off” indicating that the reference data is not tobe updated. On the other hand, in the compressed packet Ph(5), the valueof the flag Ih5 is “On” indicating that the reference data is to beupdated. That is, after transmission of the compressed packet Ph(5), thereference packet identifier is updated to (ID=1) which indicates thecompressed packet Ph(5), and the reference data is updated to thetransmission data (D5) corresponding to the reference packet identifierPh(5).

Accordingly, an identifier Ih1 indicating “compressed”, a referencepacket identifier (ID=1) Ih2 b, a reference data updation flag Ih5, andother header information Ih3 are stored in the header section Hph ofeach of the four compressed packets Ph(6)˜Ph(9) which follow thecompressed packet Ph(5). Further, difference data (D5−D6), differencedata (D5−D7), reference data (D5−D8), and difference data (D5−D9) arestored in the data sections Dph of the compressed packets Ph(6)˜Ph(9).

In the compressed packets Ph(6)˜Ph(8), the value of the updation flagIh5 in the header section Hph is “Off” indicating that the referencedata is not to be updated. On the other hand, in the compressed packetPh(9), the value of the flag Ih5 is “On” indicating that the referencedata is to be updated. That is, after transmission of the compressedpacket Ph(9), the reference packet identifier is updated to (ID=2)indicating the compressed packet Ph(9), and the reference data isupdated to the transmission data (D9) corresponding to the referencepacket identifier Ph(9).

Accordingly, an identifier Ih1 indicating “compressed”, a referencepacket identifier (ID=2), a reference data updation flag Ih5, and otherheader information Ih3 are stored in the header section Hph of each ofthe compressed packets Ph(10) and Ph(11). Further, difference data(D9−D10) and difference data (D9−D11) are stored in the data sectionsDph of the compressed packets Ph(10) and Ph(11), respectively.

In the compressed packets Ph(10) and Ph(11), the value of the referencedata updation flag Ih5 in the header section Hph is “Off” indicatingthat the reference data is not to be updated.

The uncompressed packet Pg(1) and the following compressed packetsPh(2)˜Ph(11), which have been transmitted from the transmitting end, aresequentially received at the receiving end in the normal datatransmission state, and the transmission data (D1)˜(D11) correspondingto the respective packets are restored.

To be specific, the difference data (D1−D2), (D1−D3), (D−D4), and(D1−D5) of the packets Ph(2), Ph(3), Ph(4), and Ph(5) are restored withreference to the transmission data (D1) of the uncompressed packet Pg(1)which is identified by the identifier (ID=0).

Further, difference data (D5−D6), (D5−D7), (D5−D8), and (D5−D9) of thepackets Ph(6), Ph(7), Ph(8), and Ph(9) are restored with reference tothe transmission data (D5) of the compressed packet Ph(5) which isidentified by the identifier (ID=1).

Further, the difference data (D9−D10) and (D9−D11) of the packets Ph(10)and Ph(11) are restored with reference to the transmission data (D9) ofthe compressed packet Ph(9) which is identified by the identifier(ID=2).

Turning to FIG. 22, it is assumed that a transmission error occurs inthe compressed packet Ph(10) during the above-described packet-by-packettransmission. In this case, when the compressed packet Ph(11) isreceived, restoration of the difference data (D9−D11) stored in thiscompressed packet Ph(11) is performed in the same way as in the casewhere no transmission error has occurred in the compressed packetPh(10).

That is, also in this fifth embodiment, as in the first embodiment, whenperforming restoration of the difference data (ΔD) stored in eachcompressed packet Ph, the reference data to be used for this restorationis not the transmission data of a packet immediately before thecompressed packet to be processed but the transmission data of anuncompressed packet which has been transmitted first or immediatelyafter occurrence of a restoration error, and the transmission data of aspecific packet which has been transmitted every time a predeterminednumber of packets was transmitted.

Therefore, in this fifth embodiment, even when a transmission erroroccurs in a compressed packet other than the specific compressed packet,this transmission error does not affect restoration of the subsequentcompressed packets which have been received normally. In this case, onlythe error packet is discarded at the receiving end, and no restorationerror notification is sent from the receiving end to the transmittingend.

When a transmission error occurs in the uncompressed packet Pg(1) or thespecific compressed packet during the packet-by-packet datatransmission, a restoration error notification is sent to thetransmitting end in the same procedure as described with respect to FIG.32. Immediately after the restoration error notification is received bythe transmitting end, an uncompressed packet is transmitted from thetransmitting end and, thereafter, the ordinary compressed packet and thespecific compressed packet are repeatedly transmitted. At the receivingend, the error packet and the subsequent compressed packets arediscarded.

Hereinafter, the operation of the data transmission apparatus 105 willbe described.

For example, when continuous transmission data (D1)˜(D11) which havebeen transmitted from a provider by a transmission method such as theEthernet (refer to FIGS. 21 and 22) are input to the data transmissionapparatus 105 as a first transmission signal S1, the reception unit 11receives these transmission data (D1)˜(D11) by the transmission method.These transmission data are sequentially output to thecompressed/uncompressed packet formation unit 12 e, as received dataSrc.

In the packet formation unit 12 e, packets for transmitting therespective transmission data to the receiving end are formed on thebasis of a transmission protocol such as PPP. At this time, any of anuncompressed packet, an ordinary compressed packet, and a specificcompressed packet is formed in accordance with a packet decision signalJp from the compression/uncompression decision unit 13, and a referencedata updation signal Jr from the updation decision unit 17. Theuncompressed packet Pg and the compressed packets Ph so formed aresequentially transmitted to the packet transmission unit 16, and thepacket transmission unit 16 transmits them as a second transmissionsignal S2 to the receiving end.

To be specific, when communication is started or when an errornotification reception signal Sn is supplied from the error notificationreception unit 14 to the decision unit 13, the decision unit 13instructs the packet formation unit 12 e to form an uncompressed packet,by the packet decision signal Jp. In cases other than described above,the decision unit 13 instructs the formation unit 12 e to form acompressed packet.

In the case where the formation unit 12 e is instructed to form acompressed packet, the formation unit 12 e forms a specific compressedpacket as the compressed packet when the reference data updation signalJr indicates updation of the transmitting-end reference information Im1,and it forms an ordinary compressed packet when the signal Jr does notindicate updation of the reference information Im1.

In the compression/uncompression decision unit 13, it is decided thateither an uncompressed packet or a compressed packet is to be formednext, on the basis of the compression/uncompression identifiers Ih1 ofthe respective packets which have been formed, and the errornotification reception signal Sn. Then, a packet decision signal Jpindicating the type of the packet to be formed is output. To bespecific, immediately after starting communication, a packet decisionsignal Jp indicating that an uncompressed packet is to be formed isoutput, and when an error notification reception signal Sn is input, apacket decision signal Jp indicating that a compressed packet is to beformed is output.

In the packet formation unit 12 e, formation of an uncompressed packetPg is performed in the same manner as described for the firstembodiment. Further, a compressed packet Ph is formed on the basis ofthe transmitting-end reference information Im1 (i.e., identifier (ID)and reference data (D)) which is stored in the reference informationmanagement unit 15 e. At this time, in the header section Hph of theordinary compressed packet Ph, the reference data updation flag Ih5 thatis set at “Off” is stored together with the compression/uncompressionidentifier Ih1, the reference packet identifier Ih2 b, and the otherheader information Ih3. In the header section Hph of the specificcompressed packet, the updation flag Ih5 that is set at “On” is storedtogether with the compression/uncompression identifier Ih1, thereference packet identifier Ih2 b, and the other header information Ih3.In the data sections of these compressed packets, the difference data(ΔD) based on the reference data managed by the reference informationmanagement unit 15 e are stored.

Further, when the uncompressed packet Pg or the compressed packet Ph isformed, in the reference information management unit 15 e, theidentifier (ID) and the corresponding reference data (D) as thetransmitting-end reference information Im1 are updated to the referencepacket identifier (D) for identifying the packet Ph or Ph and thecorresponding transmission data (D), on the basis of a transmitting-endupdation control signal Cm1 supplied from the packet formation unit 12e.

Further, in the reference information updation decision unit 17, thenumber of the ordinary compressed packets which have been transmittedafter transmission of the uncompressed packet or the specificcompression packet is counted on the basis of thecompression/uncompression identifiers Ih1 of the respective packets, andthe reference data updation flags Ih5 of the compressed packets. Whenthe count reaches a predetermined value (in this case, 3), a referencedata updation signal Jr is output, and the count is reset. When thecount is smaller than the predetermined value, no reference dataupdation signal Jr is output.

The process steps performed by the packet formation unit 12 e will bedescribed hereinafter, with reference to a flowchart shown in FIG. 23.

When the transmission data received by the reception unit 11 is input tothe packet formation unit 12 e (step Sc1), the packet formation unit 12e inquires of the decision unit 13 about the type of a packet to beformed next, i.e., either an uncompressed packed or a compressed packet(step Sc2), and the type of a packet to be formed is decided on thebasis of the packet decision signal Jp from the decision unit 13 (stepSc3).

Based on the result of the decision, when an uncompressed packet is tobe formed, an identifier (ID) for identifying this uncompressed packetis given to this packet as a packet identifier Ih2 a, and anuncompressed packet Pg including the packet identifier (ID) is formed(step Sc11). Thereafter, the identifier (ID) and the correspondingreference data (D), which are stored as the transmitting-end referenceinformation Im1 in the management unit 15 e, are updated according to aninstruction from the packet formation unit 12 e (transmitting-endmanagement control signal Cm1) (step Sc12).

On the other hand, when a compressed packet is to be formed, the packetformation unit 12 e inquires of the reference information managementunit 15 e about the identifier (ID) and the reference data (D) which arestored as the transmitting-end reference information Im1 in themanagement unit 15 (step Sc4). Then, the data section Dph of acompressed packet is formed on the basis of the identifier (ID) and thereference data (D) obtained by the inquiry (step Sc5).

Thereafter, the packet formation unit 12 e inquires of the decision unit17 as to whether the transmitting-end reference information Imi is to beupdated (step Sc6), and it is decided whether the information Im1 is tobe updated on the basis of the reference data updation signal Jr fromthe decision unit 17 (step Sc7).

When the transmitting-end reference information Im1 is to be updated,the identifier (ID) and the reference data (D) stored in the referenceinformation management unit 15 e are updated according to an instructionfrom the packet formation unit 12 e (step Sc8), and a specificcompressed packet Ph is formed (step Sc9).

On the other hand, when the transmitting-end reference information Im1is not to be updated, an ordinary compressed packet is formed (stepSc10).

Then, those packets formed as described above are transmitted to thetransmission unit 16 (step Sc13).

Thereafter, the packet formation unit 12 e returns to the process ofstep Sc2. The above-mentioned process steps are repeated until the lastpacket is transmitted.

Next, a description will be given of the operation of the data receptionapparatus 205 when plural packets are sequentially transmitted as shownin FIGS. 21 and 22.

In the packet reception unit 21, the packets Pg(1) and Ph(2)˜Ph(11)which have been transmitted from the transmitting end are sequentiallyreceived, and the received packets are input to the error packetdetection unit 22. In the error packet detection unit 22, when it isconfirmed that the received packets have been normally transmitted,these packets are output to the packet restoration unit 23 e as normalpackets Pno. However, when it is not confirmed that the received packetshave been normally transmitted, the received packets are discarded aserror packets. Although this fifth embodiment employs CRC (CyclicRedundancy Check) as an error detection method, the error detectionmethod is not restricted thereto.

In the packet restoration unit 23 e, it is decided whether the normalpacket Pno (normally-received packet) is a compressed packet or anuncompressed packet, according to the compression/uncompressionidentifier Ih1 included in the header section of the normal packet Pno.

For example, when the normally-received packet Pno supplied to therestoration unit 23 e is the uncompressed packet Pg(1), the restorationunit 23 e restores this uncompressed packet Pg(1) by taking thetransmission data (D1) from the data section Dpg.

Next, in the management unit 25 e, the identifier (ID) and the referencedata (D) which are stored as the receiving-end reference information Im2are updated according to the receiving-end management control signal Cm.Thereby, the identifier (ID) and the reference data (D) stored in themanagement unit 25 e are updated to the identifier (ID=0) and thetransmission data (D1), respectively. Thereafter, the restoration unit23 e outputs the transmission data (D1) as restored data to the outputunit 26, and the output unit 26 outputs the transmission data (D1).

On the other hand, when the packet supplied to the restoration unit 23 eis the specific compressed packet Ph(5), the restoration unit 23 einquires of the reference information management unit 25 e as to whetherthe reference packet identifier (ID=O) and the corresponding referencedata (D1) which are included in this compressed packet are stored in themanagement unit 25 e. When the identifier (ID=O) and the data (D1) arestored in the management unit 25 e, the restoration unit 23 e restoresthe transmission data (D5) of this packet by using the reference data(D1) and the difference data (D1−D5).

When either the reference packet identifier (ID=0) or the correspondingreference data (D1) is not stored in the management unit 25 e, thereceived compressed packet is discarded as an error packet, and an errorsignal Se indicating occurrence of a restoration error is output to theerror notification transmission unit 24. On receipt of the error signalSe, the error notification transmission unit 24 sends a restorationerror notification signal Ne to the transmitting end.

Further, in the restoration unit 23 e, the reference data updation flagIh5 of the header section Hph is checked. When this flag Ih5 indicatesthat the receiving-end reference information Im2 is to be updated, areceiving-end updation control signal Cm2 is output to the referenceinformation management unit 25 e. In the management unit 25 e, accordingto the control signal Cm2, the stored identifier (ID) and thecorresponding reference data (D) are updated to, for example, thereference packet identifier (ID=1) and the reference data (D5).Thereafter, the transmission data (D5) restored in the restoration unit23 e is output to the output unit 26, and the output unit 26 outputs thetransmission data (D5) as a signal S3.

When the packet inputted to the restoration unit 23 e is an ordinarycompressed packet, the processes to be performed in the restoration unit23 e and the reference information management unit 25 e are identical tothose described for the first embodiment, except the process of checkingthe reference data updation flag Ih5.

Hereinafter, the process performed by the packet restoration unit 23 ewill be described with reference to a flowchart shown in FIG. 24.

When the normally-received packet Pno is supplied from the error packetdetection unit 22 to the packet restoration unit 23 e (step Sd1), it isdecided whether the normally-received packet Pno is an uncompressedpacket or a compressed packet (step Sd2).

When the normally-received packet Pno is an uncompressed packet, theuncompressed packet is restored, i.e., the transmission data (D) istaken from the data section Dpg of this packet Pg (step Sd12). Then,according to an instruction from the packet restoration unit 23 e, theidentifier (ID) and the reference data (D) which are stored as thereceiving-end reference information Im2 in the management unit 25 e areupdated to the packet identifier of the uncompressed packet and thecorresponding transmission data, respectively (step Sd13). Thetransmission data taken from the data section of the uncompressed packetis sent to the output unit 26 (step Sd9).

When the normally-received packet is a compressed packet, the packetrestoration unit 23 e inquires of the reference information managementunit 25 e as to whether the reference packet identifier (ID) Ih2 b andthe corresponding reference data (D) which are included in thecompressed packet are stored in the management unit 25 e (step Sd3), andit is decided whether the identifier (ID) and the data (D) are stored inthe management unit 25 e (step Sd4).

When the reference packet identifier (ID) Ih2 b and the correspondingreference data (D) are not stored in the management unit 25 e, thenormally-received packet is discarded as an error packet (step Sd10),and an error signal Se is output to the error notification transmissionunit 24 (step Sd11). Thereafter, the packet restoration unit 23 ereturns to the process of step Sd2.

On the other hand, when the reference packet identifier (ID) Ih2 b andthe corresponding reference data (D) are stored in the management unit25 e, the difference data of the compressed packet is restored to thetransmission data on the basis of the reference data (step Sd5).

Next, it is detected whether the reference data updation flag Ih5 storedin the compressed packet indicates that the reference information is tobe updated (step Sd6). When the flag Ih5 indicates that the referenceinformation is to be updated, the identifier (ID) and the reference data(D) which are stored as the receiving-end reference information Im2 inthe management unit 25 e are updated (step Sd8). Thereafter, thetransmission data is output to the output unit 26 (step Sd9). When theflag Ih5 does not indicate that the reference information is to beupdated, the transmission data is output to the output unit 25 withoutupdating the receiving-end reference information Im2 (step Sd9).

Thereafter, the packet restoration unit 23 e returns to the process ofstep Sd2. The above-described process steps are repeated until the lastpacket is received.

As described above, according to the data transmission method of thefifth embodiment, when performing packet-by-packet data transmission byusing uncompressed packets in which uncompressed transmission data arestored, and compressed packets in which compressed transmission data arestored, the transmission data of each compressed packet is compressed byusing, as reference data, the transmission data of an uncompressedpacket or a specific compressed packet which has been transmittedpreviously to the compressed packet. Therefore, so long as theuncompressed packet or the specific compressed packet is normallytransmitted, even when a transmission error occurs in some compressedpacket, difference data of the compressed packets which have beennormally transmitted after the error packet can be restored by using thetransmission data of the uncompressed packet or the specific compressedpacket. Therefore, the number of the compressed packets to be discardeddue to the transmission error is significantly reduced. As the result,the quality of data transmitted in the radio section is improved. Inother words, the effective rate of data transmission is increased, andthe time and cost required for transmission of unrestorable packets aresignificantly reduced.

While in this fifth embodiment, the timing for updating thetransmitting-end or receiving-end reference information (i.e., totransmit one specific compressed packet every time three compressedpacket are transmitted) is decided at the transmitting end, the specificcompressed packet may be transmitted every time a predetermined period(n sec.) has passed, or when the size of the difference data stored inthe compressed packet exceeds a predetermined threshold.

Further, the specific compressed packet may be transmitted when thetransmitting end receives a request for updating the reference data fromthe receiving end, or when the size of difference data (or the averageof difference data) exceeds a predetermined threshold.

For example, the transmitting end transmits the specific compressedpacket when it receives a request for transmission of the specificcompressed packet from the receiving end.

Further, the transmitting end transmits the specific compressed packetwhen the size of compressed data included in the compressed packet to betransmitted to the receiving end exceeds a predetermined value.

Furthermore, the transmitting end transmits the specific compressedpacket when the average of sizes of compressed data included in thecompressed packets to be transmitted to the receiving end exceeds apredetermined value.

The method of deciding the timing to update the transmitting-end orreceiving-end reference information may be a combination of theabove-described methods.

The following effects are achieved by deciding the reference dataupdation timing as described above.

Usually, a difference in video data, audio data, or header informationbetween two adjacent packets is very small or 0 in many cases, but adifference in such data between distant packets tends to be large.Therefore, by periodically transmitting an uncompressed packet, thequality of data transmitted by radio is improved, and the average ofdifference data is reduced, that is, the compression efficiency of thedata section is improved.

(Modification of Embodiment 5)

While in the fifth embodiment the compressed data to be stored in thedata section Dph of the compressed packet Ph is the difference data (ΔD)between the whole transmission data of the compressed packet and thewhole transmission data of the uncompressed packet Pg, the compresseddata to be stored may be obtained by compressing a part of thetransmission data of the compressed packet Ph.

That is, the transmission data is separated into a plurality ofcompression target data corresponding to different items to becompressed, and non-target data which is not to be compressed. In thedata section Dph of the compressed packet Ph, difference data betweenthe compression target data of the transmission data corresponding tothe uncompressed packet and the compression target data of thetransmission data corresponding to the compressed packet is stored asitem-basis compressed data, and the non-target data of the transmissiondata corresponding to the compressed packet is also stored in the datasection Dph.

FIG. 25(a) shows the data structure of an uncompressed packet Pi whichis used when transmission data is composed of compression target data(data to be compressed) and non-target data (data not to be compressed).

The uncompressed packet Pi is composed of a header section Hpicontaining header information, and a data section Dpi containingtransmission data (D) to be transmitted by PPP, as uncompressed data Ir.The header section Hpi is composed of a compression/uncompressionidentifier Ih1 indicating whether the data in the data section iscompressed or not, a packet identifier (ID) Ih2 a for identifying thisuncompressed packet, and other header information Ih3. The uncompresseddata Ir is composed of four pieces of item-basis target data Ira, Irb,Irc, and Ird corresponding to four items to be compressed, andnon-target data Inc which is not to be compressed. In FIG. 25(a),item-basis transmission data (Da), (Db), (Dc), and (Dd) are stored asthe item-basis compression target data (item-basis uncompressed data)Ira, Irb, Irc, and Ird.

FIG. 25(b) shows the data structure of a compressed packet Pj which isused when transmission data is composed of compression target data andnon-target data.

The compressed packet Pj is composed of a header section Hpj containingheader information, and a data section Dpj containingpartially-compressed data (ΔD) to be transmitted by PPP. The headersection Hpj is composed of a compression/uncompression identifier Ih1, areference packet identifier (ID) Ih2 b, a reference data updation flagIh5, and difference data existence flag Ih6, and other headerinformation Ih3. The difference data existence flag Ih6 indicateswhether compressed item-basis target data which is not “0” is includedin the compressed packet or not.

The data section Dpj includes four pieces of item-basis compressed dataIda, Idb, Idc, and Idd corresponding to four target items to becompressed, and non-target data Inc which is not compressed. Thecompressed data Ida is a difference (item-basis difference data (ΔDa))between the item-basis transmission data (Da) of the transmission datacorresponding to the uncompressed packet and the item-basis transmissiondata (Da) of the transmission data corresponding to the compressedpacket. The compressed data Idb is a difference (item-basis differencedata (ΔDb)) between the item-basis transmission data (Db) of thetransmission data corresponding to the uncompressed packet and theitem-basis transmission data (Db) of the transmission data correspondingto the compressed packet. The compressed data Idc is a difference(item-basis difference data (ΔDc)) between the item-basis transmissiondata (Dc) of the transmission data corresponding to the uncompressedpacket and the item-basis transmission data (Dc) of the transmissiondata corresponding to the compressed packet. The compressed data Idd isa difference (item-basis difference data (ΔDd)) between the item-basistransmission data (Dd) of the transmission data corresponding to theuncompressed packet and the item-basis transmission data (Dd) of thetransmission data corresponding to the compressed packet.

In this case, in the data transmission apparatus 105, the referenceinformation management unit 15 e tables the reference packet identifier(ID), the data indicating each target item to be compressed, and thereference compressed data corresponding to each target item (item-basisreference data), and stores them as transmitting-end referenceinformation Im1.

Likewise, in the data reception apparatus 205, the reference informationmanagement unit 25 e tables the reference packet identifier (ID), thedata indicating each target item to be compressed, and the referencecompressed data corresponding to each target item (item-basis referencedata), and stores them as receiving-end reference information Im2.

FIG. 25(c) shows the process of forming a compressed packet Pj(Y) bycompressing transmission data D(Y).

In this case, in the reference information management unit 15 e, areference packet identifier (ID=X) and item-basis reference data(Da(X)), (Db(X)), (Dc(X)), and (Dd(X)) are stored as thetransmitting-end reference information Im1.

Further, the respective item-basis difference data (ΔDa), (ΔDb), (ΔDc),and (ΔDd) are differences between the respective item-basis referencedata (Da(X)), (Db(X)), (Dc(X)), and (Dd(X)) as the transmitting-endreference information Im1 and the corresponding item-basis transmissiondata (Da(Y)), (Db(Y)), (Dc(Y)), and (Dd(Y)) in the transmission data(D(Y)), as represented by the following formulae (1) to (4).ΔDa=Da(X)−Da(Y)=0  (1)ΔDb=Db(X)−Db(Y)≠0  (2)ΔDc=Dc(X)−Dc(Y)≠0  (3)ΔDd=Dd(X)−Dd(Y)=0  (4)

Since the values of the item-basis difference data (ΔDb) and (ΔDc) arenot 0 while the values of the item-basis difference data (ΔDa) and (ΔDd)are 0, the difference data existence flag Ih6 in the header section Hjof the compressed packet Pj(Y) is set at “On” indicating that there areitem-basis difference data which are not 0 amongst the plural item-basisdifference data, and only the item-basis difference data (ΔDb) and (ΔDc)are stored in the data section Dj of the compressed packet Pj(Y).

FIG. 26 shows a part of the compressed packet Pj(Y).

Further, each item-basis difference data includes, as common information(format), a following difference data existence flag and a referencedata type flag. The value of the following difference data existenceflag Ico1 in the compressed data Idb (item-basis difference data (ΔDb))is set at “On” indicating that another item-basis difference data isstored after this item-basis difference data (ΔDb) in the data sectionDpj, and the value of the reference data type flag Ico2 in thecompressed data Idb indicates that the item-basis transmission data (Db)should be referred to when restoring the item-basis difference data(ΔDb). Further, the value of the following difference data existenceflag Ico1 in the compressed data Idc (item-basis difference data (ΔDd))is set at “Off” indicating that no compressed data follows thisitem-basis difference data (ΔDd) in the data section Dpj, and the valueof the reference data type flag Ico2 in the compressed data Idcindicates that the item-basis transmission data (Dc) should be referredto when restoring the compressed data Idc.

Since each item-basis difference data includes the following differencedata existence flag Ico1 and the reference data type flag Ico2, only theitem-basis difference data whose data quantity is not 0 can be includedas components of the difference data in the data section of thecompressed packet.

Therefore, the transmission data can be compressed for each component ofthe transmission data (data corresponding to each target item to becompressed), whereby the storage area (e.g., a RAM) of thetransmitting-end reference information management unit 15 e or thereceiving-end reference information management unit 25 e can be reducedwhile maintaining the compression efficiency.

Further, in the item-basis difference data Idc, as peculiar information(format) different from the common information (the following differencedata existence flag and the reference data type flag), difference datalength information Ium1 and a compression method type flag Inu2 arestored.

The difference data length information Iun1 shows the data size of theitem-basis difference data Idc, and the compression method type flagInu2 is used for specifying a method for restoring the item-basisdifference data Idc from plural restoration methods.

Since the difference data length information Iun1 is included in theitem-basis difference data (ΔDc), when the item-basis difference data(ΔDc) is relatively small, the data size can be reduced, whereby thecompression efficiency is further improved.

Further, the compression method type flag Iun2 comprises, for example,2-bit data, and the compression efficiency is further improved bypredetermining compression methods according to the values.

For example, when the value of the compression method type flag Iun2 is“00”, the item-basis difference data is a difference (ΔDn) from thereference data. When the value is “01”, the item-basis difference datais (ΔDn/2). When the value is “10”, the item-basis difference data is(ΔDn/8). When the value is “11”, the item-basis difference data is(ΔDn/64).

FIGS. 27(a)-27(c) are diagrams for explaining specific data to be storedin the uncompressed packet Pi and the compressed packet Pj. FIG. 27(a)shows data to be transmitted (transmission data) by these packets, andFIG. 27(b) shows transmission data in the uncompressed packet anddifference data in the compressed packet. Here, transmission of RTP datais taken as an example.

The above-described transmission data corresponds to an IP packet(RTP/UDP/IP data) Pipb shown in FIG. 29(d), and the transmission datacomprises data corresponding to first to fourth compression target itemsK1 to K4. The data corresponding to the first and second target items K1and K2 (compression target data Ira and Irb shown in FIG. 25(a)) are anRTP packet's sequence number (SN) and a time stamp (ST), respectively.The data corresponding to the third target item K3 (compression targetdata Irc shown in FIG. 25(a)) is an IP packet's identifier (ID), and thedata corresponding to the fourth target item K4 (compression target dataIrd shown in FIG. 25(a)) is an UDP port number. The specific datacorresponding to the respective target items in the respectivetransmission data (D1)˜(D5) are shown in No. 1˜No. 5 on the table ofFIG. 27(a).

When the RTP/UDP/IP data are actually transmitted as the transmissiondata (D1) to (D5), the transmission data are stored in PPP packets(uncompressed packet and compressed packet) according to PPP (Point toPoint Protocol), and the PPP packets are transmitted from the datatransmission apparatus 105 to the data reception apparatus 205.

At this time, the compression target data Ira, Irb, Irc, and Ird of thetransmission data (D1) are stored without being compressed, in the datasection Dpi of the uncompressed packet Pi(1). Further, in the headersection Hpi of this packet Pi(1), a compression/uncompression identifierIh1 (1 bit), packet identifiers (ID) Ih2 a and Ih2 b (5 bits), and otherheader information Ih3 (not shown in FIG. 27(b)) are stored.

Further, in the data sections Dpj of the compressed packets Pj(2) andPj(3), the item-basis compression target data (item-basis uncompresseddata) Ira, Irb, Irc, and Ird corresponding to the respective targetitems of the transmission data (D2) and (D3) are compressed and storedas item-basis compressed data Ida, Idb, Idc, and Idd. The item-basiscompressed data Ida corresponding to the sequence numbers (SN) of thecompressed packets Pj(2) and Pj(3) are 8-bit difference data “1” and“2”, respectively. The item-basis compressed data Idb corresponding tothe time stamps (ST) of the compressed packets Pj(2) and Pj(3) are16-bit difference data “50” and “100”, respectively. The item-basiscompressed data Idc corresponding to the IP packet identifiers (ID) ofthe compressed packets Pj(2) and Pj(3) are 8-bit compressed transmissiondata “1” and “2”, respectively. Further, the item-basis compressed dataIdd corresponding to the UDP port numbers of the compressed packetsPj(2) and Pj(3) are 0 bit, respectively.

Further, in the header section Hpj of each of the compressed packetsPj(2) and Pj(3), a 1-bit compression/uncompression identifier Ih1, a5-bit reference packet identifier (ID) Ih2 b, a 1-bit reference dataupdation flag Ih5, a 1-bit difference data existence flag Ih6, and otherheader information Ih3 (not shown in FIG. 27(b)) are stored.

In the data structure shown in FIGS. 26 and 27(a)-27(c), the referencedata type flag Ico2, the difference data length information Iun1, andthe compression method type flag Iun2 are included in the differencedata (ΔDn). However, these data may be added to the header sections Hpi,Hpj or the data sections Dpi, Dpj when the reference data is updated,i.e., when the uncompressed packet Pi is formed or when the specificcompressed packet Pj (compressed packet having the reference dataupdation flag “On”) is formed.

In this case, the transmitting-end reference information management unit15 e and the receiving-end reference information management unit 25 eare constructed so as to manage the difference data length informationIun1 and the compressed method type flag Iun2 for each item-basisreference data, whereby, at the receiving end, restoration of compressedpackets can be performed on the basis of the information Ium1 and theflag Iun2.

In this case, the compression efficiency is further improved because itis not necessary to add the difference data length information Iun1 andthe compression method type flag Iun2 in the item-basis difference data(ΔDn) of the compressed packet every time the compressed packet istransmitted.

The above-described method of adding the reference data updation flagIco2, the difference data length information Iun1, and the compressionmethod type flag Iun2 to the header section or the data section of theuncompressed packet or the specific compressed packet when theitem-basis reference data is updated, is especially effective in thecase where the compression method for transmission data changes atregular intervals among a plurality of complicated compression methods,and higher compression efficiency is expected.

While in this fifth embodiment a data transmission method usinguncompressed packets, specific compressed packets, and ordinarycompressed packets is described for packet-by-packet data transmission,data transmission may be performed by switching the method between themethod of this fifth embodiment and another data transmission method,according to the transmission status of packets.

In this case, as the second data transmission method, any of the datatransmission method according to the first to fourth embodiments or thedata transmission method using the V. Jacobson's header compressionmethod (refer to FIG. 31) may be employed.

1. A data transmission method for sequentially transmitting data inunits of packets each containing transmission data from a transmittingend to a receiving end, said method comprising: transmitting anuncompressed packet in which predetermined transmission data is storedas uncompressed data at regular intervals; subsequently continuouslytransmitting compressed packets in which at least a portion oftransmission data following the predetermined transmission data iscompressed and stored as compressed data; and forming compressed datathat is to be stored in any packet other than an uncompressed packet,based on transmission data of the uncompressed packet and transmissiondata of a packet to be compressed.
 2. The data transmission method ofclaim 1, wherein the uncompressed packet containing the sametransmission data is continuously transmitted a plurality of times tothe receiving end.
 3. The data transmission method of claim 1, furthercomprising: receiving from the receiving end, based on an occurrence ofa restoration error of the uncompressed packet, a notificationindicating an occurrence of the restoration error; and transmitting theuncompressed packet in response to the notification.
 4. The datatransmission method of claim 1, wherein each compressed packet containsa sequence number that indicates the position of the respectivecompressed packet in a sequence of the compressed packets which havebeen transmitted after the uncompressed packet.
 5. A data receptionmethod for receiving data in units of packets each containingtransmission data from a transmitting end to a receiving end, saidmethod comprising: storing predetermined transmission data as anuncompressed data packet at regular intervals; subsequently continuouslyreceiving compressed packets in which at least a portion of transmissiondata following the predetermined transmission data is compressed andstored as compressed data; and restoring transmission data of acompressed packet to be restored, based on transmission data of theuncompressed packet and compressed data included in the compressedpacket to be restored.
 6. The data reception method of claim 5, furthercomprising continuously receiving the uncompressed packet containing thesame transmission data a plurality of times.
 7. The data receptionmethod of claim 5, further comprising discarding, when a restorationerror occurs, only an error packet.
 8. The data reception method ofclaim 5, further comprising: transmitting, when a restoration error ofthe uncompressed packet occurs, a notification indicating an occurrenceof the restoration error to the transmitting end; and receiving theuncompressed packet, which is transmitted from the transmitting end, inresponse to the notification.
 9. The data reception method of claim 5,wherein each compressed packet contains a sequence number that indicatesthe position of the respective compressed packet in a sequence of thecompressed packets which have been received after the uncompressedpacket is received.
 10. A data transmission apparatus for sequentiallytransmitting data in units of packets each containing transmission datafrom a transmitting end to a receiving end, said apparatus comprising: atransmission unit operable to transmit an uncompressed packet in whichpredetermined transmission data is stored as uncompressed data atregular intervals, and then to continuously transmit compressed packetsin which at least a portion of transmission data following thepredetermined transmission data is compressed and stored as compresseddata; and a compression/uncompression section operable to perform acompression process of forming compressed data that is to be stored inany packet other than uncompressed packet, based on transmission data ofthe uncompressed packet and transmission data of a packet to becompressed.
 11. The data transmission apparatus of claim 10, whereinsaid transmission unit is further operable to continuously transmit theuncompressed packet containing the same transmission data a plurality oftimes to the receiving end.
 12. The data transmission apparatus of claim10, further comprising: an error notification reception unit operable toreceive a notification indicating an occurrence of a restoration errorfrom the receiving end, based on an occurrence of the restoration errorof the uncompressed packet, wherein said transmission unit is furtheroperable to transmit the uncompressed packet in response to thenotification.
 13. The data transmission apparatus of claim 10, whereineach compressed packet contains a sequence number that indicates theposition of the respective compressed packet in a sequence of thecompressed packets which have been transmitted after the uncompressedpacket.
 14. A data reception apparatus for receiving data that aretransmitted in packet units from a transmitting end, said apparatuscomprising: a reception unit operable to receive an uncompressed packetin which predetermined transmission data is stored as uncompressed dataat regular intervals, and then to continuously receive compressedpackets in which for each compressed packet at least a portion oftransmission data following the predetermined transmission data iscompressed and stored as compressed data; and a restoration unitoperable to restore transmission data of a compressed packet to berestored, based on transmission data of the uncompressed packet andcompressed data included in the compressed packet to be restored. 15.The data reception apparatus of claim 14, wherein the uncompressedpacket containing the same transmission data a plurality of times. 16.The data reception apparatus of claim 14, wherein said apparatus isoperable to discard only an error packet when a restoration erroroccurs.
 17. The data reception apparatus of claim 14, furthercomprising: an error notification transmission unit operable to transmita notification indicating an occurrence of a restoration error to thetransmitting end when a restoration error of the uncompressed packetoccurs, wherein said reception unit is further operable to receive theuncompressed packet transmitted from the transmitting end in response tothe notification.
 18. The data reception apparatus of claim 14, whereineach compressed packet contains a sequence number that indicates theposition of the respective compressed packet in a sequence of thecompressed packets which have been received after the uncompressedpacket is received.